Depends on the pads. I once got less than 2000 miles from a set of stock organic TRP pads and that was awfully short. Went to Shimano semi-metallic pads and get closer to 8,000 miles. I also use TruckerCo sintered pads and they last around 10,000 miles possibly a little more if I refrain from riding in the rain. As for using up all your pads in one ride? I posted the explanation below recently on another group:
Most (all?) mechanical disc calipers cannot accommodate wearing through an entire set of pads without the safety nub on the arm contacting the caliper body - this serves as a stopping point so the caliper does not overrun the pad piston/actuator bearings/actuator arm inside and fail. You will run out of braking ability but most of the pad will be left. You can see an example here (direct PDF link) on page 15 - all mechanical disc brakes produced in recent years have this feature in some form. If you are curious how mechanical disc brakes work, this exploded diagram of paul klampers illustrates the basic principle: Link
A few times during gravel events in the rain, I will have to stop and adjust the barrel adjuster to the max and give the fixed side a few turns to get adequate braking power. On a rainy 60 mile gravel race today my brakes were worn away to the point that I could pull the level to the bars and only get minimal braking, even after adjusting to max pull right before the safety stop on the caliper arm. Thankfully the course was mostly flat and not terribly curvy. There is still at least 70% of the pad remaining, I reseated the calipers and adjusted the fixed side but did not need to change the pads.
I will also add, make sure you pair your pads to the right rotor. Not all rotors are compatible with all pads and a mismatch can cause one, the other or both to wear much faster than expected.