Multi-cutter blade blunt in 1 day. Bad blade? Bad Technique?
2 hours of intermittent cutting on one blade is pretty good.

If those blades are naff, then all of the ones I've used in the past (Bosch, Porter Cable, Dewalt, Imperial, off-brand no-name) are also naff.

The only thing I could suggest is keeping the blade clear of bitumen/ wood/ etc. Vacuuming up sawdust while you're cutting can help.

They've managed to convince us this is acceptable because these tools are so incredibly useful.