WEP in a P-51 or Spitfire, for example, was just running the engine at a higher boost level. There was a slight increase in the chance of engine failure, but it did mess up engine maintenance schedules. The is one account of a Spitfire MK V pilot panicing and running his aircraft at WEP for about 30 minutes with no adverse affect on the engine. Pratt & Whitney ran an R-2800 (P-47, F4U, F6F, ect) engine at WEP power for more than 24 hours in a factory test.
For aircraft that used an additive for WEP (Fw190s, later Bf109s, P-47s, ect), WEP would only last as long as the additive did.