My DX4000 has twin filter cartridges, each covered in the normal brown paper filter bag, and also a seperate filter cloth that acts before the cartridges/bags. I only ever take the cloth out and beat it to clean, never had to replace the rest as they appear still to be virtually as new despite several years of hobbyist use.
The DX1500 (?) I have though doesn't have the cloth pre-filter and there the paper bag needs cleaning off with every emptying session and then replacing every now and then when worn/torn. The cartridge again hasn't needed changing itself.
DOn't know if any of that helps?!!!
Cheers, Paul