At my peak, I had several years where I did between 94 and 98 gigs each year. (As you might guess, I'm full-time.)
I used to have a theory:
Given that there's 52 weeks per year, that gives a mobile DJ a total of 104 weekend days (Fridays/Saturdays) to book. (Not including weekday club work.) Being the maximum, I equate 104 gigs with the speed of light.
Any Fridays or winter days that might NOT get booked might be offset by the occasional Sunday or weekday gig that will sometimes crop-up, so our light speed remains constant. (I never have broken the 104 mark.)
For many years, I approached, but never quite reached that ultimate goal. In fact, the more I advertised, the more calls I'd get for dates I was already booked! It got to the point where I was referring more work to other DJs than I was getting myself! Thus the other end of the analogy: it would take an infinite amount of advertising to achieve gig "light speed."
These days, though, I'd be happy with half that number. When Michigan's economy went in the toilet, so did the local DJ industry. Coupled with the current "do-it-yourself" craze, where people are renting sound systems and playing iPods at their events, DJs who are surviving aren't getting what they used to for their services. I sincerely hope that what's happening in Michigan doesn't spread.