Park hopping is a flat fee regardless of the number of days on a ticket - $52+tax ($55 and change in total).
My general advice is that if you are unsure if you need it, don't get it until you need it. If you manage to get through your trip without actually needing to park hop, then you've saved yourself some money.
If you are certain you'll need it, add it in advance if you can (save yourself a trip to a ticket booth) or at the earliest convenient opportunity so you can take advantage of the option at the earliest.