Really depends on the individual field and runway... at my home field we have one runway that you can comfortably turn final at 1000' on QFE and one that you need to be closer to 400'.
Personally, I try and have a height benchmark closer to the threshhold and start looking for that on base leg. That way I have an idea about when to start descending before I get that 'never-going-to-get-it-down-better-go-around' feeling
Hersh