I asked my vet, and she told me that it is almost impossible for cats to OD on catnip. That is, she didn't know of cats suffering any ill-effects form it even if they have unlimited access.

I grow my own catnip outside, and I just clip off a leaf or two per cat per day and feed it to them fresh. When my cats are around potted plants, they tend to pull them out by the roots so I don't allow them access to the whole plant.