Did my first cosmic and I think once is probably enough.
1-20 wasn't that bad.
21-40 was also manageable.
41-50 was pretty frustrating.
51-55 made me really regret doing it in the first place.
56-60 was just mind numbing.
Took 6 hours total boosted to around 700-800% the entire time. If I were to do it again, I would make sure to save the story missions til the end as they made 52-55 pretty fast.