Mine have catnip toys (homemade so I know there is a lot of catnip in them) and will generally 'know' when they have had enough and walk away.
If I give them catnip itself, they tend to fight and I hate trying to separate them so they rarely get any plain 'nip anymore.
So as far as too much, as long as your cat doesn't think so, I would guess it is ok, but then, it depends if your cat is playing / eating it all the time or whether they self regulate also.
Daphne hasn't developed a nose for nip yet. Seb loves the stuff. I put out some loose nip for him and he rolls around in it for awhile, gets silly and then sleeps. He'll go back to it a few times until its all gone