I've only done a couple of organised rides (not "sportives" as such, as there was no timing, but not audax as routes were signed and marshalled). One was 40 miles away, one was 80'ish. I have one more pencilled in this year, and that's 100 miles away. Reasons for choosing were distances (60, 75 and finally 100 mile routes), calendar dates (about 6 weeks apart), and crucially for me they were circular routes. I would travel further for "special events", but generally I'd keep it down to a max of 1 1/2hrs drive each way. And I've no real interest in huge fields of riders either, so go for smaller events.