How to exit meditation?
I disagree with Sumida and ChrisW, an hour is too long for beginners and experts alike. In Zen monasteries we do zazen for between 30 - 45 minutes followed by about 10 minutes of kinhin (fast or slow walking meditation), then another period of zazen. For a beginner 20 minutes (or even less) is fine. In time you should aim to raise it to around 30 mins. If you want to do more you should break it up with kinhin (or possibly some yoga).

I agree with Chris that you should use a timer (a countdown timer on a digital watch is ideal). When the timer goes off, then get up (some people sway gently from side to side before getting up). You shouldn't be pondering during meditation whether to get up or not, a) you shouldn't be pondering things during meditation and b) if you practise like this you will turn meditation into an aesthetic experience (I'm enjoying this so I'll sit longer/I'm not enjoying this so I'll get up) which is not right meditation.