My party is level 17 and one of my PCs is a sorcerer.
He discovered a combo using the extended spell with time stop to just freeze time. Time stop lasts a minimum of 2 rounds or 12 seconds so by using 13 sorcery points he could freeze time for 24 h. He could then use 8 of those 24 hours to take a long rest regaining his sorcery points and the 9th level spell slot. He could then do whatever he wanted and just before time unfreezes he could recast it, rinse and repeat over and over again.
Unless he affects another creature, he can keep time frozen for as long as he chooses. I'm not so worried about the potential balance issues of this because I can introduce scenarios and enemies specifically tailored to counter this ability (homebrew if you were wondering), but I do wonder if this freezing time strategy is viable by official and Adventurers League rules?