It is possible to clean a HEPA filter. This is by using a can of compressed air, which you will use to blow the dust. You can also use water from the water supply and filter gently using a garden hose. Get detailed information on http://www.ehow.com/how_5078988_clean-hepa-filters.html.
I've never seen any instructions on how to clean them so you can re-use them. The particles they capture are very small, and they use electrostatic means to hold on to them.
I use a HEPA filter mask, and they say if you store a used mask for a long time, and the filter gets mold, you can place the filter in water that has just been taken off the boil, then let them dry, but I don't think this cleans them from the dust.
I reckon you have to replace them when they become a dark color.