Bluetracker
Tracks Blizzard employees across various accounts.
How long do challenge rift chests last in the "mail"?
I was wondering if I can complete them weekly and save up for the start of the next season to pop them but I'm not sure if the mail has a duration to claim it (like the amount of time you're alloted to claim seasonal -> nonseason stash items after season end).
Nevalistis
30 days. Keep in mind that only Challenge Rift rewards that were earned during the Season can be opened on those Seasonal characters. They cannot be saved over to new Seasons or earned before the Season starts.