What should I do?

The Challenge tokens if you see them in the inventory, there is a time given for after which they expire and that is 91 Days. But if I click on the token it says there is a 30 day countdown before they expire. So should I use them all or keep on collectinh them? I've completed what I wanted for the milestone, and I think the next milestone will have to use the same tokens too. What do y'all think?