Seal Treasure is locked to 1x Seal Scroll use until you use 20 (or use 20 free draws). Since it's a way for helping begginers get some new ninjas on server start.
The other treasures are locked to level progression, being last treasure - GNW Trasure, unlocked when you reach level 70.

It's recommended though, to wait for Kage treasure to unlock (around level 50 I think, or 60), before spending seal scrolls, since best value for your seal scrolls are Kage and GNW. Also it is recommended to use seal scrolls when there's a Rebate going on, so you can get good return on investment.