You have 60 days in total to get the code for each Mystery Box that you earn on Mode. One week prior to that expiration date, you’ll also get a reminder so that it won’t happen to you that you actually lose out on that great opportunity to earn $EARNM coins.
However, there is no good reason to not create the code for your Mystery Boxes right the minute you earn them and enter them into the dApp (yes - you’ll spend a few cents for revealing the Mystery Boxes, but that’s it). The Sweepstakes entries or Music Earning boost you get along with your Mystery Box (depending on where you live) will not get lost by creating that code and revealing them on the dApp. But in turn, as soon as you get them onto the dApp they start vesting. If you keep them on the Mode Earn App without revealing them on the dApp, they will sit there without gaining in value. So what are you waiting for 🙂