I like to make it performance specific (making it clear I'm referring to THIS particular performance). If they know you're talking about this particular performance, you can sometimes get a longer gag out of it.
Spec: How loing have you been doing magic?
Me: Uh, 2minutes and 13 seconds. Why do you ask?
Spec: No, I meant how long have you been performing?
Me: I just told you, it's been 2 minutes and . . . well now it's 25seconds, since we started.
Spec: No, I meant how long have you been practicing?
Me: I practiced before I got here, this is the actual show. . .
Spec: I mean, How long have you been interested in magic?
Me: My interest varies from day to day, it's sad really. I happen to have ADD, which of course stands for Attention Deficit - that's a nice shirt, where'd you get it?
If you can keep the wordplay going, it's a great gag; and it allows you to avoid the long drawn out conversations that tend to follow the "How long have you been doing magic?" question.
That is really funny lol![/b]