A better rule of thumb is miles because hours don't reflect the amount of energy put into your drive train.

Typical recommendations are every 500-750miles for a mountain bike. For the average mountain biker I think that would be more like every 125 hours at 6 mph. Mountain bike chains see much more abuse than road, and you can go further too.

You can be more scientific by using a chain ware tool to measure how much stretch there is.