I think muscle memory can last for years. I started lifting again after 4 years off and I am really making good gains now. I started with 135 for around 10 reps on bench. I think that a lot of the initial strength gains made are from getting your CNS back in shape, because my lean bodymass is higher now than it was when I would use 225 four years ago. I can only guess that it had something to do with gaining 10% more bodyfat, combined with that muscle fullness development.