A disk might last 10 minutes before it is counter productive to keep using it .
They don't go bad and you will need them later or inside or topside etc.
You are looking to get an even surface , dull with no marks.
Keep the sander flat and don't push down on the sander , if you have to push against the hull and hear a major change in rpm's ,the paper is dull and needs changing .
Tipping the sander wears out the pad that holds the disk in place , been there done that .
I get 25 disks for $5 so don't be cheap , you are wasting time, not saving paper.