I'd say anywhere from 40k onwards, but with a healthy dose of 'depends'.
You have to remember that a rotary is designed to wear out and need tips replacing etc, it's the very reason why it works. Add boost to that and it accelerates wear, turn the boost up and it's even worse. A stock RX7 (if such a thing even exists any more!) is probably good for at least that. Start adding boost or tuning and you're going to be reducing that considerably. Short trips as well, much more wear on a rotary on a short trip than on a piston engine. As such, it wouldn't be unrealistic to see a rebuild being needed from just 20k, if it hadn't been looked after.
If you can find an engine that's just been rebuilt, I'd buy it and run it without worrying too much as I'm guessing you're like me and don't keep your cars too long. If it hadn't been done, then I'd simply budget a couple of grand in to get it done before I put any miles on it.