Had to pick carrots in the poll because it totally depends upon the complexity of the song. Throw me a I, IV, V, progression, tell me the key, tell me to listen for the changes and the turn around and I'll have it down by the second verse of the song we are performing. Other songs I may have to spend an hour or two to get it down.
Getting the bass line down though and singing lead on it at the same time often takes a lot of repetition. So that can stretch out your practice time. I never did put in enough practice to play the bass for that Nirvana's "Come As You Are" while singing it. I keep screwing up the bass line on the verse. The bass line is not hard, but I have trouble singing it and playing it at the same time. So, it would take me a while to get that bassline to go automatically while I'm singing the song. "Team Spirit" on the other hand is dead simple to sing and play at the same time.
Even taking the singing out of it though, you're going to find that different songs take different amounts of practice to get them down. Don't know that I could actually make a generalization about how long it takes.