There are people who just don’t have time to play 100+ waves in one chunk of time.
Just as with monolith progress now being saved, I suggest adding a postpone/continue options for Arena.
So after every 5 waves, you’d have “postpone” option along with “claim rewards” and “fight harder waves”. It will return you to The End of Time.
Next time you should have an option to continue fighting, starting with the waves where you left. And without spending arena key.
I can’t think of an easy way how the continue option should be implemented, maybe just add dialog option for that happy brawler NPC nearby, rather than redesigning the arena key reciever.