Recollect once, inbound HK following two sectors and long delays, we were told to land enroute for crew change. I had calculated that we were not going to exceed 3 hrs discretion but, following an hour arguing with ops, I said "Soddit!" and routed across Burma at an intermediate level, unable to contact any ATC unit, to land and crewchange.
I DID once go over three hours and the snottogram my boss received from the CAA made it clear that there would be 'consequences' if it happened again.