The more skill speed or spell speed you have the more she adds. She has a flat % buff. It doesn't JUST help DPS it also helps healers, by the way. I can't give you hard numbers because I'm a lazy SOB, but I have come across charts before that show DPS output for all classes+what extra DPS can be pumped out with selene. Most classes get a decent boost. SMN doesn't really get much out of selene though.