Tim Walker: With a work rate this slow, no wonder Leonardo was good

Tales from the Water Cooler: The painter's patron must have felt like Abramovich waiting for Torres to score

When I read that it was going to be the cultural event of the year, I naturally pre-booked my tickets for the National Gallery's Leonardo da Vinci: Painter at the Court of Milan exhibition well in advance.

Though previously aware that Leonardo was far from prolific, I must confess I was surprised to find there'd be just nine authenticated da Vinci paintings in the exhibition – or eight, depending on your opinion of the newly attributed Salvator Mundi (that's "Stoner Jesus" in English), from an existing 14 (or 15).

In 50 years – during which he revolutionised portraiture; invented the helicopter; posited a theory that would become known, 400 or so years later, as plate tectonics; and drew "Vitruvian Man", thus launching 1,000 T-shirt designs – Leonardo supposedly started just 20 paintings.

Scarcity is, of course, a boon to an aspiring genius's brand image: JD Salinger published one short novel and a handful of short stories, Stanley Kubrick made a movie every three-and-a-half years. Had they done more, they might have produced some chaff to go with the wheat.

But let's look at the other side of that coin: Leonardo started 20 paintings in a 50-year career – some of which remain unfinished, and at least two of which are of Exactly The Same Thing. That's two-and-a-half years per painting. So, no wonder they were good.

Ludovico Sforza, Leonardo's Milanese patron, must have felt like Roman Abramovich waiting for Torres to score. Not all of us have wealthy dukes paying our rent while we faff and procrastinate. If we did, the world would likely contain a few more masterpieces.

With that much time on his hands, maybe Rolf Harris could have revolutionised portraiture, too.