I read somewhere, a long time ago that the rule of thumb was:
1 minute keep rpms <= 1,000 (idle)
2 minutes keep rpms <= 2,000 (driving)
3 minutes keep rpms <= 3,000 (driving)
4 minutes keep rpms<= 4,000 driving
etc
When I had cars that had oil temp and water gauges, I'd pay close attention to...