General rule of thumb is 20-30 minutes with an hour in between to 'rest' so to speak. Think of it this way, take your hand and squeeze your wrist pretty tight and flex your hand. You will notice that all the blood vessels will become larger as the blood flows in but not out. After awhile your hand will feel numb too. This is how a ring works, now if the blood can not flow out after a time the pressure from the heart will build to a point where the vessels burst.
Now, if the ring is properly fitted the rule should be one to live by. If it is loose it will allow blood to flow past so longer wear can be had. Also, if you are just wearing it when not erect there is not really a limit. I have worn a stainless ring for six hours with no problems. Just don't wear one when sleeping as your penis will get hard without you knowing it.
Lastly, as with all things. If you feel any pain, discomfort or see any discoloration remove the ring asap.