How many feet of pipe are allowed between pull boxes?

There is no limit in the NEC. This distance is limited by your ability to pull wire the distance needed. If you install 100 feet of pipe, you will need a way to pull wire that distance. If you install 300 feet of pipe, same thing.

Fish tapes come in various lengths but can be pricey and rarely used if you buy a long one. There are ways to use a shop vac to suck or blow string in the conduit and for the most part length doesn't matter using this method. If the conduit is large enough and/or the wire big enough you can use this string to pull in a rope with the strength you need.