I guess 400'/1500' are the reference for most. Of course, these are minimum heights, not mandatory!
I'd check with a performance expert before getting too carried away with that assumption. The reason you clean up after takeoff is to get the aircraft into a lower drag configuration; if you leave it 'dirty' for too long, with a lower rate-of-climb, you may not get the terrain clearance assumed in the perf. calculations later on in the flight profile... As I said in a previous post, this is only likely to become apparent at limiting weights (and after a V1 cut) but if you want to stay on the right side of FAR & JAR...