This is a strange question to ask. I have known _how to program_ since I was in the 7th grade when we were taught ahem gbasic back in 1996. I have learn't C in my undergrad, taught myself a bit of python during the same period. Upto this point programming was recreational trying to solve trivial problems (I learnt python primarily to solve project euler problems).

It wasn't until I started my masters, 5 years ago, that I did any serious projects and 2 years ago that I did professional work.

So when people ask how long have you been programming for, what is it they are looking for ?