And these concert hall marathons are nothing compared to what's been happening in the record shops: CD cycles just finished or still in progress include Schubert Lieder and sonatas, Purcell church music, Britten folksong settings, Haydn symphonies and quartets, Shostakovich film scores, the piano works of Grieg (in 13 volumes) and Liszt (in 70), the 32 symphonies of Havergal Brian... One company has just announced the complete set to end all complete sets: all the concertos of Vivaldi - so now the very patient (and very rich) will be able to discover whether the creator of the hardy perennial Four Seasons really did write "the same concerto 600 times". A recent New Yorker cartoon summed it all up rather neatly - just a billboard, with the words "All the Telemann you can stand".
Well, who's complaining? After all, you don't have to go to all the concerts, or buy all the CDs. And a complete cycle does at least mean that we can hear such rareties as Bruckner's Second Symphony, Sibelius's Third, Nielsen's Sixth or Vaughan Williams's Antartica in concert. As for recordings, Hyperion's Purcell surveys, Collins's Britten folksongs, ASV's Haydn quartets and Andras Schiff's extra-complete Schubert sonatas (including all the fragments as well as the finished works) on Decca have rescued some wonderful things from near-complete obscurity. Then there's the BIS "Complete Sibelius" series, which we have to thank for bringing to light the theatre music and the fascinating original versions of the Violin Concerto, En Saga and the Fifth Symphony. And the influence of pioneering sets like Karl Bohm's complete Mozart symphonies or Antal Dorati's Haydn marathon has been - and still is - immense.
But rule out genuinely worthwhile projects like these and a huge quantity remains. Record shop shelves are creaking under the weight of all those fat, glossy display cases and their compendious booklets. Isn't there just a hint of madness in all this? Some performers (or is it their record companies?) now seem just about incapable of selective recording. It has to be the complete set - not just the Beethoven piano sonatas you particularly appreciate and have experimented with often enough in concert, but all 32, preferably in quick sequence. Gone are the days when Sir Thomas Beecham could record only the Beethoven symphonies he really admired, or Herbert von Karajan could leave out Sibelius's Third because he felt he didn't understand it.
Gone? Well, not entirely. There are still rare individualists, like Sir Charles Mackerras, who will record just three Beethoven symphonies, politely acknowlege the critics' and record buyers' approval and then move on to something else. Otherwise, there seems to be enormous pressure from somewhere to go for the set. Perhaps some of that pressure is coming from the consumers. Nowadays, we are told, most shoppers prefer to get everything they want from superstores rather than trudge up and down the high street with their heavy bags, comparing quality and prices. A complete set of Beethoven symphonies saves a lot of agonising, especially when it's offered at bargain price by a record club in your Sunday magazine. And there's at least one attraction for the companies: if you have a nicely packaged complete Beethoven to offer a club or retail-chain, you might recover your losses quicker than you expected.
But the news from the shops seems to be that the big "popular" sets aren't selling. So is the pressure coming from the performers? There must be a kind of macho appeal for some - the idea of conquering a whole chain of mountains rather than the odd peak. But the metaphor doesn't stretch very far: a mountain climbed is a mountain climbed, full stop; but you can perform or record a symphony or a sonata with no more than minimal understanding. The chances of one performer or group of performers cultivating equally profound understanding in all nine Beethoven symphonies, all 21 of Schubert's completed piano sonatas or all 83 Haydn (or "attrib Haydn") string quartets is virtually nil - especially when artists seem to feel they have to complete their cycles in double-quick time. Hyperion's decision to divide its huge Schubert Lieder project (now at volume 25 of a projected 35) between several singers is commendable, but all too rare.
Perhaps what we are witnessing is a craving for context. For some - and not just the compulsive collectors - there seems to be a feeling that the individual work of art is no longer enough on its own, or at least that it grows in meaning when you hear it as part of a larger process, or as a stage in a kind of "macro-work". Certainly, concert series can bring occasional shafts of enlightenment - is that a prophetic pre-echo of Beethoven's Ninth in the slow introduction to his Second Symphony? But, in my experience, such enlightenment comes more often across the genres - Beethoven apparently rehearsing for his "Ode to Joy" theme in the Choral Fantasia, and perhaps in the finale of his Fourth Piano Concerto, or re-working his original theme in the finale of the Op 132 String Quartet (after another instrumental recitative!).
It was exactly this kind of cross-genre comparison that made the BBC's recent Ives weekend so rewarding. Stay the course in most cycles and you can just as easily end up suffering from fatigue - "the attention retires," as Dr Johnson put it. Worse still, you can end up thinking less of a composer. I'm sure I would admire Martinu's Sixth Symphony even more if I didn't know that he'd used that same radiant plagal cadence in well over a dozen other works (including his Symphony No 5). And, for me, there are ideas in the first movement of Shostakovich's Eighth that sound uncomfortably close to things at equivalent points in his Fifth - in such cases, ignorance is definitely closer to bliss.
And what is "complete" anyway? The fact that a composer died when he/she did doesn't mean that the life's work had reached its natural conclusion. You can see Beethoven's Ninth as the culmination of the cycle, but Beethoven had made some headway with a 10th before his death - what would that have told us about the Ninth? Is his last string quartet best understood as "the last", or is it the start of something new - a move away from the style of the four preceding "late" quartets towards . . ? How complete is our understanding of Mahler if, like too many conductors, we ignore the almost-finished 10th Symphony, so authoritatively restored by Deryck Cooke? What would Dvorak think of modern "complete" sets of his symphonies, which invariably include the four early works he seems to have been happy to forget? And how complete is a Bruckner symphonic cycle that includes only one version of each of the numbered symphonies, as the Barbican's recent Bruckner-Mozart Series did?
Yes, it is wonderful to be able to hear the Cinderella works, and perhaps even to discover that they can be more enchanting than their musical sisters. But wouldn't it be nice if, just once in a while, they got a chance to go to the ball on their own, and not in the inevitable family party?
n Sibelius Symphony Weekend: Nos 3 & 7, 7.30pm tonight; Nos 1 & 4, 7.30pm Sat; No 2, 4pm and Nos 5 & 6, 8pm Sun. Barbican (0171-638 8891)