How high is up?

It totally depends on your environment. Some people ride in areas where there's a ton of broken glass, or on trails where there are goathead thorns. Those people can flat multiple times per week.

I can go a year between flats. I just flatted last week, picked up a drywall screw. Before that it was quite a few months. I think I average twice a year.

If you're in an area with lots of road hazards, you can run kevlar-belted tires. That probably won't stop a drywall screw though.