The limiting factor of how long can you use combat/WEP should be for the same reason as in RL, the temperature of inter cooler. The engine the P-47 had, had no problem to WEP and combat power on test stands with "infinite endurance" intercooler for long times with no adverse effects. Once the discharge air was too hot or you ran out of water, you had to reduce throttle to get back inside the safe range for the rated fuel (150oct when???).
Therefore the WEP and other engine modes durations should be modeled by the inter cooler discharge temperature and not some arbitrary value that is based on secret arts of timers.