Programmers - people who write computer code for a living - talk about this a lot and suggest that the half-life in their field is about 18 months. That's why they're always buying books about new programming languages, always going to conferences, always subscribed to lots of different mailing lists.
They assume that 50 per cent of their knowledge becomes stale, flat and unprofitable every year-and-a-half, so they have to keep replacing it, constantly adding new skills and mastering new tools.
And, crucially, they've developed great learning habits, they've learned how to learn, how to admit there are things they don't know and how to be constantly developing new skills without having to take weeks off work to do it. They're interesting people to hang out with - always looking for new things to learn and new ways to learn them.
Thinking about which had me wondering what the HalfLife of Knowledge might be in advertising and communications. I don't really know, but I bet it's sped up recently.
Certainly some will bang on about how the skills of advertising are timeless: it's all about persuasion and story-telling, blah, blah, blah, and I guess that's true.
But at least half the skills are incredibly specific - how to tell stories with particular tools X and Y, how to persuade in channels A and B - and those specifics have switched from changing as slowly as media to changing as rapidly as technology.
It took us a while to work out how to use TV properly, we've still not cracked it with the web and we don't seem to be getting any faster. So we could do worse than look at technology companies to show us how to do it, to teach us how to learn. They devote explicit resource to it with proper, ring-fenced, R&D budgets. They know how to prototype effectively - lots of small bets, some of them crazy. They know that failure is necessary and they're happy to talk about it.
And, corporately, we need to devote less time to training people how to get better at things they already know and more time to teaching them things that are completely new. But, personally and individually, we need to learn from programmers and get the learning habit.
Frankly, it's an indictment of our lack of curiosity and surfeit of sloth that there are companies training executives on how to use Twitter and Facebook. Regular people learn how to use these things themselves; we should be capable of the same.