Time Away From Gym: Any Day 09:00-12:00
Rematch Call Window: Friday Morning
"The eleventh gym leader, Lt. Surge, is missing from the gym most days between 9am and 12pm. He is found outside the Power Plant after you have captured Zapdos. Talk to him and he'll ask to see a Pikachu. Show him the Pikachu and he'll give you his number. Then, on Friday Morning, you can arrange a rematch."
You can rematch every gym leader, look at the link to see how and when.