As the others have noted, there are a lot of variables. The largest gain will come through aerodynamic changes in riding posture. The more aggressive and aerodynamic friendly your road riding posture is, the less air you have to push out of the way. Riding narrower handlebars and in the drops as much as possible would also decrease the resistance.
This of course can vary from frame to frame and rider to rider, with TT genre bikes probably being the pinnacle.
Setting all that aside, I don't think it would be unrealistic to expect ~ 20% + gain.