Assuming, and I believe not unreasonably, that she rests on the Sabbath, every other Saturday and for, say, two days at Christmas and one day at Easter, plus a further 10 days at Blackpool or Balmoral, she would be hard at it for no fewer than 274 days a year. Again, assuming that the advice given to each of 'the people from all over the world who were concerned about their health', merited some degree of thought and careful consideration, to allocate five minutes per response would not seem particularly excessive. On this basis, 30,000 letters per year divided by 274 days multiplied five minutes per letter equals nine hours letter-writing per day.
Again, assuming no breaks for leg-stretching or trips to the loo - which is pretty hard on a 91-year- old - and that a 10-hour working day is not unreasonable, this leaves only one hour per day for book-writing, or approximately 12 hours per book. Having never read one of Dame Barbara's books, I do not know whether 12 hours seems reasonable or not, but whichever way you look at it, her levels of productivity ought to be an example to us all.
JOHN A. HEYWOOD