WEP is only time based in that the heat rises steadily and hits its limit at that 5 (or 10) minute WEP duration. It is the HEAT itself that turns off WEP. Think of it as a constantly regenerating power bar in an arcade game. You use it, it drains. It will still steadily drain, but stop using it and it starts replenishing. So you get 5 minutes continuous, or more than 5 minutes with a break in between.
The heat dials on the P-38 probably come into play when you lose a radiator and the engine overheats. You can go into the red on planes with WEP and keep an eye on it, turn the engine off, etc... Don't recall how that works in the 38G, though.