IMO, "enough is enough" whenever you or he is ready to quit. I know the common practice in pool is to NEVER quit ahead, but that's just absurd. One of the many reasons I left pool for poker is there is never any issue with the timing of one's quiting. With the minor exception of winning a big hand within the first few orbits and immediately leaving, you are free to come and go as you please. In poker, knowing when to quit while you're ahead is just as important as knowing when to quit when you are behind. If I have been 8+ hours, winning a good amount, and I become tired enough my focus suffers, I quit the game without even the first wimper from other players.
Pool IMO should be the same way. If you have been playing a few hours, and are ahead, if you are tired and will likely start playing less optimally, or you're just not interested in playing any longer, you should be free to quit at any time. Of course it would be polite to offer to play another day, and you should honor such an offer. Or, if you agreed to freeze up an amount of money, or play a certain amount of sets, you should also honor this.
If you're feeling guilty you're taking too much money from someone, and that bothers you enough you want to quit, IMO, you should be able to quit. I personally would likely keep playing in this situation, but your personal values are just that....personal.
The age old never quit while ahead in pool is likely something drummed up because someone was losing and knew they would likely be able to outlast the other person stamina wise and get their money back. Its a terrible "etiquette" that is almost never seen in any other aspect of gambling.