It's going to vary a lot depending on the amount/type of road debris that you encounter, your tires, and the age of your tires.

I've had two flats within the last year, both on the same day, and within 500 yards of each other. I had noticed a few days before that the tires were starting to look a little worn. I should have swapped them out right away but put it off and paid the price.

Something similar happened to another set of tires I had. They were great for two years, - no flats at all. Then I got two or three flats within a couple of weeks and I knew it was time for them to go.

Some tires have flat protection built in. Especially with skinny, high pressure tires, this can make a big difference. The stock tires on my road bike had no flat protection and it seemed like I averaged about a flat a month with those.