1/4s and 1/2 sheets are different sizes to different people.
If you match the cake to the board & box sold as that size you will need:
1 cake mix for a 12x8; approx. 20 minutes at 325 degrees
2 mixes for a 11x15 ; approx. 30-40 minutes @ 325
2 1/2-3 for a 12x18; approx. 45-60 minutes @ 325
Each of those sizes is for a cake/pan 2" deep.
Time # are just a guide. You should bake them until you smell them, then test w/a toothpick or touch lightly w/you finger near the center; if it springs back any it is done. A well baked cake should not pull away from the edges of the pan. You should take it out before that