It all depends on things like the road surface you're riding on. I get flats every 40-50 miles, but the surface I ride on has tons of these thorn bearing burrs. Even though I am on a vigilant watch, flats still happen. I just put a liner in my tube, so hopefully that'll help some. Flats are an expected part of doing biking for me.
In the past, I've gone on surfaces that didn't have these thorns (at least not as much) and I might go several hundred miles without a flat. Personally, I'd be happy with an average of one flat per 100+ miles right now.