Its pretty speratic for me.
Sometimes Ill go two weeks without listening to a single Tupac song, other times that is all I listen to for two weeks straight. Furthermore, ill sometimes mix it up: some Pac, and a variety of other things (like some Wu Tang, other west coast shit, OutKast, etc etc).
I listen to almost all of my music in the whip, so its not like I listen to it for hours on end. Ill just keep something in there until I feel like putting a different cd in. The last two days, ive been playing 7 Day Theory non-stop but up until then I hadnt listen to that album in over a month.