the amount of comping is totally dependent on the project/artist. I've spent blocks of 19 hours comping the living shit outta a load of awesome takes...cos the artist wanted it that way. This was syllable by syllable and every piece of articulation/breath etc.....
That was wrong..but it was also exactly right for how the artist wanted to sound, they could hear all the differences and it kinda makes up the sound they are known for. I dont like that sound personally, but hey, it's their party
If its something I'm producing, I love to just get one, first, keeper take and then patch it from maybe one other take if theres a major problem somewhere. The reality is that there are hardly any singers who can do that, so I usually try to get three good takes and if it isnt happening by then, take a break and maybe return to the vocal the next day..You know its gonna be a mission if after three takes you haven't got anything... I try to avoid loop recording as much as possible as I would rather fix a performance, than try and create one from disjointed sections. If the singer is really poor and theres no hope of a performance or anything really cool even with the best vibes and encouragement, then its time to break out auto-tune
or melodyne, hacking and slicing, ghost singers and finally spending a goodly while in comp-city.