We often see people who sleep 4-5 hours and they feel rested; and also we often see people who sleep much more but they wake up as unrested and dissatisfied. Therefore, is is not possible to say that there is a straight relation between a good leep and sleep duration
Although some of us sleep as soon as putting our heads on the pillow, some of us start to think what did we do in the day or the problems may happen the next day. We hardly sleep after several distressed hours. So, the length of time taken to fall asleep and our level of distress during this time is one of the key factors that make us satisfied from our sleep.
A second factor that allows us to wake up rested is the the number of waking ups in the night. It has been shown by researches that; short or long term sleeps in the night - called as electrical or behavioral - cause "fragmented" sleep whose continuity is broken; and this prevents us to perceive the sleep as satisfactory.
Although data related to sleep latency and sleep continuity are important criteria regarding the satisfaction of sleep; it is not identified yet a definite and unique criteria which shows that we slept well.