This is the problem with a zillion cheap tat shop vacs. Pleated filter sticking straight down into the bin. They load up between the pleats in no time, the pleats get battered where they are in line with incoming debris, and brushing the pleats out is a horrid job done in a cloud of dust.
A two or three motor numatic vac uses at least a two layer fabric filter instead of the pleated thing which doesn't load up the same way and is far easier to clean. Weighted donuts in the fabric filter cause it to drop down when the vac is switched off performing a very simple, basic filter shake.
A last detail to look out for is does a vac have bypass motors ? These have a separate supply of clean air to cool the motor instead of relying on the dirty air after it's been through the filter for cooling. If you run a non bypass motor choked up for a while (typically an orbital sander session) they tend to overheat the motor and trip them out. Premium shop vacs don't suffer this problem as they have the right type of motor.
