Too tired would be when you can't even hold your eyelids open for more than a few seconds. Like when you look at someone and it looks as though they were heavily inebriated however they are just that tired. That's when you are too tired.

A good time to go to sleep is whenever your body is finally ready to settle down. If you lay in bed for a long amount of time without falling asleep you should get up and go do something for a bit and try again later. If you keep laying there it teaches your brain to become used to the practice of laying in bed for a few hours and then falling asleep. Or at least I think that sounds right don't quote me on that.