At NWA as a block/line holder, you have a monthly min and max. You can drop anything, but you can't go below your monthly min. If you are on the high side, you can drop a higher value trip and pick one up on the open board worth less time. This month, our min is 72 hrs and mx is 84 hrs. If you want, you can go up to 5 hrs above your max (your choice, scheduling can't make you do it).

Like rickair said, there has to be enough reserve coverage to cover your dropped trip. If there is'nt, you don't get to drop it.