As people have pointed out, it completely depends on your body. However, a good measure to see whether you're fully rested is to be aware of your energy levels

As people have pointed out, it completely depends on your body. However, a good measure to see whether you're fully rested is to be aware of your energy levels after you wake up, and to be aware whether you feel sleepy/sleep deprived (not the I ate too much sleepy) during the middle of the day. I've found them to be good indicators to whether i need to catch up on my sleep.