see this is one of those big fat questions in which the only real answer is, IT DEPENDS! every person is different. many of the specific types and sources and labs in terms of each drug are different. every pct is a little different. in general, if your committed and you really want to keep your gains, if the work ethic is there then yes you will keep almost all your muscle gains with some leaner bulk/cutting cycles, this cant be said for heavy bulking cycles where you pile on 15+lbs of water and fat. I know lots of guys who run light var cycles and gain 6 or 7lbs at a time and put a couple dozen pounds on their big 3 lifts and they keep 90% of that, same goes with guys who do test cycles with proper pct.
personally, i find the best way to hold onto my gains is to keep the diet and supplement intake high while tapering off the gear, then adding pct drugs (im always trying different stuff) as i start to taper off, increasing the dose of the nolva, clomid, arimidex or whatever it may be for usualy half of the total cycle duration after cesation. (12 week cycle =6 week pct) then i usually take a few months off. now, with hard training, even if you loose some of your gains post cycle, give it a couple months and you will back up pretty close to where u were on cycle (depending on the cycle of course) in terms of strength, ive done this often, and have alotta buddies who also go natty for months at a time and still make good gains. old school juicers often point out that this is a good sign that your natural test production is still going and working properly too (at least when it comes to training that is)