I will try not to get to indepth or long winded here,
WEP stands for WAR EMERGENCY POWER, it is available in differing amounts depending on the plane type.
Ki-84 - has great WEP, and it recovers quickly, when I say recover, I am talking aout the time the temp hand goes from ye//owred to green/yellow...before it is fully replenished.......
The 109's & 190's also have a better WEP/Boost than most other planes have..... the BOOST in the 109's %& 190's is simulating a nitrous boost , where as say the F4U series or F6f is water injected type boost......
you are only limited by the time it takes for your WEP ( boost ) to recover, and you can use it even if it has only recovered a few seconds, but it is soon used up again.......
not all planes have WEP , the a6m2, F4f, yak for example do not, where as the Fm2 does.......etc.....
hope this short answer helps..
as noted by DREDger, you have to be at 100% throttle to use wep, 99% throttle wep will not come in/on........how ever it will make your lower RPM settings if used for cruising, kick back to full/100%
also, some planes show it going into the RED zone on the temp gauge a good ways, some show it cutting out right at the yellow/red area of temp gauge...