In either park, it's a 10-15 minute walk from the furthest corner back up to the front gate. In IOA, Hogwarts is the furthest corner, while in USF it's the Fear Factor experience (but Diagon Alley is pretty close to that corner of the park.) We found that if the wait for the Hogwarts Express was more than about 15 minutes, it was faster to walk from one Wizarding World to the other.
Obviously this is contingent on your walking speed (me and my kids walk about 3 mph) and the crowd levels. Special events like the parade will also slow this down considerably.