Cash game poker and tournament poker are two entirely different beasts. In tournament poker you have a start, middle and end. In cash game poker the action never ends so unless you are Phil Laak, you need to create your own start and end, the middle will take care of itself.
When choosing what time to start it helps if you have a little bit of Intel on the game in question. Just because the tables open at 20.00 doesn’t mean you need to be knocking at the door at 19.59. You may want to start when you know the ale has had time to settle in and the table will be full of drunks. You may want to arrive smack on 20.00 because you need to secure your seat. Having the local knowledge will help you make your decision when to start a cash game. Fans of Betfair online poker should bear this in mind.
Choosing to finish a cash game is a whole different proposition. Unless you are under strict orders from the other half to be home at a certain time things can go awry. If you are winning big then you feel supremely confident and think the heater will last all night. If you are breaking even then your brain will tell you to sit put until you get a profit under your belt. If you are losing your mortgage your brain will tell you to stay put until you recover your losses. http://poker.betfair.com/en/how_to_play/
The longer you play, the more tired your brain becomes and the more likely you are to make mistakes. So it is always good practice to set yourself a set amount of hours that you are going to play and then leave when your times comes.
Always remember to clock off.