Same here...I place a blanket over it so that it doesn't get dirty often. The blanket gets changed and washed often. Luckily LO hasn't had any blowouts or spit up in it either.
I haven't washed it aside from the initial time. LO doesn't spit up much (a refreshing change from our experience with DD1), and she hasn't had a blowout in it yet. I'll probably throw the liner in next time I do a wash just because.
We haven't washed it yet and unless something happens to it like pee or poop, I don't see me washing it before we get rid of it. He only sleeps in it...no parties or anything.
Re: Fisher Price Rock and Play
Edit: we wash it between kids but not weekly while using it.