This depends on how many hikes you plan to do and how much of the park you'd like to see. At a minimum you will want to go to the both Emerald Pool trails, Angel's Landing (if you're not afraid of heights), and Temple of Sinawava. To do these, I'd recommend a minimum of 2 days. If you plan on doing any special hikes with permits, this could easily expand to 4 to 5 days.