Yes, there is a minimum dosage.
From "Harry Potter and the Deathly Hallows", Chapter Twenty-Five, "Shell Cottage":
There was problem after problem to overcome, not least of which was that their store of Polyjuice Potion was greatly depleted.
'There's really only enough left for one of us,' said Hermione, tilting the thick mud-like Potion against the lamplight.
As for the exact amount required, that isn't made clear in either the movies or the books. The closest there is to any mention of a specific dosage is in Chamber of Secrets (Chapter Twelve) where they each take a "large dollop" of potion.
