"Myself I typically won't start a game if I know it will have to be stopped in less than 45 minutes, and under half an hour is an absolute no-no. And once you start a game, you should play it out to the end."This is a good practice, even though I wouldn't be this strict. I sometimes start a game in a shorter time, and make if quick enough to make sure it'll be finished in time, or make sure before starting that the opponent knows I won't be able to finish it, and will agree to adjourn and go on later.
Note that it is important to discuss this with the prospective opponent before the game. Several "reasons" for adjournments are particularily insulting, unless agreed upon beforehand: to have a discussion with someone, to watch another game, or, especially, to play with someone else. None of these would be an acceptable reason for adjournment.
If you wish to stop playing for some non-reason, you can always resign.
People have even created a special word that describes this kind of net-induced delays, "netlag". Whenever someone calls netlag, you can be sure he means the traffic from his computer to the IGS and back is very slow, cases vary from 5-10 seconds per direction all way down to tens of minutes and complete connection losses. It may be useful to know that some of the biggest things causing netlag are international borders (eg. between USA and Canada) and big obstacles (Atlantic Ocean is a typical example).
Often when netlag is reasonable, people still start a game since netlag is unpredictable and can vanish totally. But quite as likely is the opposite. This has lead some people to the habit of always warning the opponent of the existence of netlag in advance, or at latest when it looks likely there will be a problem due to it, so that there would be no doubt of the cause for the need of adding time. Also, sometimes it even is necessary to adjourn the game and continue later when the network problems become less, so asking for adjournment in this context can be considered proper, and it is considered polite to accept it -- though by no means obligatory.
If you run into time problems because of your own slow play, you have only yourself to blame. Be wary of asking for too many addtimes; you may create a bad feeling. Ask for an adequate time allotment before starting the game.
Continue to "undo".