Some real world results:
There was no physical time limit on the use of 12lb boost, just a requirement to log the use and a suggestion that it be limited to 5 mins but some pilots used it for 30 mins. It did effect engine life but not severely, and later versions of the Merlin III on the Sea Hurricane were modded to allow 16lb boost and 1440hp, which was effective up to about 5500ft.