As the PP said, the park hopper is an add-on that is good for the entire length of the ticket once added. It's up to you to decide how many days you want to hop, but the price is based on the number of days on the ticket.
For example, for a 2 or 3 day ticket, the PH add-on is $49 plus tax. For longer tickets, the PH add-on is $60 plus tax. The price is based on the number of days on the ticket, not the number of days remaining. So, if you have a 6-day base ticket, the price is $60 plus tax regardless of how many days you've already used on the ticket. So if you add the PH before you've used the ticket, it would be $60. If you wait and add when you've already used 5-days, it would still be $60.