I don't see how anyone can fault someone for quitting when they are
losing, if by "losing" you mean a reasonable chunk of change, say at least
20 points.  Of course, it's no fun to set aside an evening to play
backgammon and have your opponent quit after  an hour if he or she isn't
winning but be willing to play on interminable if the reverse is true.  I
probably just wouldn't be interested in setting aside any more time for
such a person.

Some people, particularly gamblers who have some sort of imperative to win
some $ *now* may resent your quitting while you're ahead.  It's certainly
good etiquitte to give some advance warning (time or # of games) before
stopping, even with friends, but in my circles we tend not to care much.  

Incidentally, against an opponent known to steam when time is short,
"false notices" (you say, well, I gotta go in about a half hour, but then
play on for a few hours) may net you some equity.  :-)  Actually,
sometimes this isn't even insincere:  I may be wanting to go home *now*,
but if my opponent starts throwing equity at me by steaming, that may
genuinely change my willingness to play late.

Generally speaking, you should be viewing all your bg sessions as one long
session with occasional breaks for food, work, and sleep (etc.), so it
shouldn't much matter to you what the scoresheet says at the end of any
particular session.  Try to base your decision to quit on whether you are
playing decently and whether you are having fun playing.