up to 60 or so(some people say 70), however 6-11 is common while 12 is already a bit more rare (sth like 2% maybe). Everything above 12 got probably a same equal chance of i'd say 0.1% or less
Max is ten times the number of stats (str/agi/luk/...), so 70. And yes, anything over 12 is rare. Unless you're on your final re-roll of a character (see Class Changing) then it's not worth spending the time on it. I suggest taking the first 11 you get, as that's usually quick to roll.
For the most part, I wouldn't bother with trying for higher than 12. If you're lucky, a higher roll will turn up when you try it, but I know people who have tried for several days to get higher than that without success.
As Fearsyth says, max you can get theoretically is 70, but in practice it might as well be 12