The 500' is an ideal situation, however as many people have written, so much else has to be taken into account and I guess that comes from experience. Where I do most of my flying (EGGW), it is not unusual to be asked to make an early turn, for IFR traffic, especially if take off run has started from the C intersection while other traffic is using the turning circles at the end of the runway. Equally, it is not that uncommon to turn onto final at say 300' when the boke in the tower is slotting you in beween landing IFR traffic.
Incidently, I remember that Fowlmere used to have a second runway which broadly run N/S. The approach used to be over the bird sactuary and meant turning onto final at what seemed to be a very low height (200'??) As a low hours PPL, that one really used to freak me out