I would imagine that PE gains would be similar to muscle gains, i.e. if you stop using them, you lose them.
If a guy works out until he’s huge and then stops using his muscles, he will gradually shink back to size. This is the body’s way of conserving energy. Muscles require increasingly more energy as they get larger, this is why bodybuilders eat so much, whether or not they do aerobic exercise (we runners just burn off everything regardless :) ). Anyway, the body gets rid of muscles it doesn’t use to save energy. Remember, our bodies are programmed for survival in the wild, not next door to a grocery store.
I imagine PE would be similar, even if only including the muscles involved. I suppose if that is the case, then some form of edging and kegals should be enough to maintain your gains. But, if the rest of the penis reacts the same way, then other exercises would have to be continued as well.
Just my thoughts/stuff I learned in biology the other day.
MYS