The cut sheet for the faucet should detail that information. I just bought a Kohler Purist wall-mounted faucet for mounting above an undermount sink. The cut sheet says that a 1 1/2" to 6" minimum must be maintained between the bottom of the spout and the top of the fixture rim. Since I'm using an undermount sink, I'm calling the edge of the marble countertop "the edge of the fixture rim." You might also measure your current faucet and sink to give you an idea of what you find acceptable now, while staying within the guidelines given on the cut sheet. Good luck.

My faucet didn't have good instructions (a Danze which will be mounted over a vessel sink) but the standard (via the plumber who is a jerk, but knows his stuff) is six inches above the rim of the bowl (in an undermount you'd go from the counter top). The idea of six inches is that when you wash your hands, turn them sideways...six inches is about what a man's hand turned on it's side would need to wash and not hit the rim and/or faucet.