I always respond truthfully.
As of today I have been doing improv for 6 years: that's only counting the years I've been on teams and actually performing.
It doesn't really matter how long you've been doing an art form.
If I started dancing today and was amazing at it and was automatically put into the American Ballet Theatre and people asked me how long I'd been dancing and I said one day they would probable be marveled that I was such a natural.
But for some reason I've noticed performers like to exaggerate how long they've been performing.
"How long have I been singing? Ever since I was 3!"
"My parents often tell people I was singing before I even spoke my first word."
Or my personal favorite
"I was dancing the womb."
YOU WERE NOT!
You look foolish telling people you've been doing improv for 10 years when you are only 21 years old.
I remember when I had only been improvising for 1 or 2 years and I used to hate telling people because I sounded so young but if you do a good job it doesn't matter.
Adding on extra years isn't going to make you have a better show.
I don't care how long you've been doing magic, dance, singing professionally, or doing stand up.
I don't believe you anyway.