It really depends how often you play as well. 15-20 cases is a good estimate BUT, if you let the gun sit for a long time like say, over the winter, then it's best to freshen it up before playing.
Etek 3LT grey/black f/s
Valken Proton greyish/black f/s
68 4500 PE stubby, LP output f/s
68 4500 AIR tank(no reg) f/s
Creed/demon eyes f/s