I don't know how hot it has to be, but watch the temp gague in your cockpit, you can tickle the red zone but it won't last much past that.
WEP cool down time is 2-3 times longer than normal running time. I.e. 5 minutes WEP 10-15 mins cool down (for full cool down, that is).
Most planes have 5 mins WEP. It doesn't change much from plane to plane. Exceptions are:
Ki84: about 3 minutes
LW planes: 10 minutes
LA5/7: 10 minutes
Planes without WEP: no minutes.
You can run WEP til it gets hot, then wait 40 seconds and try it again, but chances are it's only dropped 5 degrees, and when you turn it back on it passes the heat limit again in 3 seconds. So you have to wait "enough" for the engine to cool that the next time you need WEP you will have enough.
I never used to use WEP much. I still don't, except in certain planes that really require it, and usually just to accelerate or zoom.