Don't have a specific temperature to tell you...nor a way to judge. In general if you can heat it till it's just below the bubbling/discoloration point I'd think that would be optimum...but getting that close could easily ruin your paint if you go over.
If you have an inconsipicuous area where if you did damage the paint it wouldn't matter you could perform a test wherein you apply a hair dryer or heat gun in a very specific and repeatable manner and see just where the paint is damaged. Then make sure you don't heat it that much when time to roll the fender well lip.
The trouble is rolling the lip is not a one time process...it should be done by bending with many small passes, bending a little each time, instead of 2 or 3 BIG bends. With the many passes the temperature of the paint will be very difficult to keep near the "optimum" point. However, many small bends (instead of a few big bends) will lessen the chance of paint cracking.
With all that being said...My advice would be to not risk it if cracked paint will be something that you cannot deal with.
Regards, Bob