the potential referred to is not intellectual or accumulative. It is a level of relationship with G-D and depends upon one's Shoresh Haneshama. To Expect Ruach Hakodesh from piety means not assessing one's potential properly. In deed there is no limit to one's maasim.However,one cannot create a temperment that another has. If you need sleep, don't try what the Vilner Gaon did.Nor should a person try to fast etc...
A good madrich for those without a Rebbe is Hilchos Dayos of the Rambam-after one has read the first 6 prakim of the Shmoneh Prakim le'HoRambam.