This is basic stuff that should already be there, a simple timer for when we activate exp\drop medals (just like the shop coupons timer).
About the effective time, it should be obvious, we have some medals that last for 1 day (brave medals) but honestly, nobody buys them because they're like "im never going to lvl for 24 straight hours lol".
If their timer stopped when logged out and restarted when logged in it would be much easier and would also move the market a bit. (and you wouldn't have people complaining cause server went down while they were on medals, even for the simple 1 hour medals)