The manufactuer of the engine will recomend a life for each type of engine they make normaly refered to as the TBO (time befor overhaul)in the UK the CAA will allow an engine to go 20% over the TBO on a public transport C of A and no restriction on a private C of A provided that it passes all the normal maintenance checks.
This may seem like a good idea to run the engine "on condition" but the life is set to the most economical overhaul time run an engine much passed the TBO and the crankshaft has a good chance of going outside overhaul limits so you have to buy a new crank at aprox $2000.
I intend to replace the engine in my aircraft at the TBO or shortly after if it comes up in the middle of the summer.