Depends on the rock itself. Some rock is really dense and some isn't, and the less dense, the better, really...
So, if you have big plans of fish, then you don't want your rock to use up all the room in the whole tank.
If you want coral, then the more rock the better, really.
But, to answer your question, no it's not too much (unless you want to try a little tang or something, and need the swimming room).