depends on what and how much your going to use it.
ive a 50ltr 2.5hp spear and jackson one.
how often is it used?
well for air tools wrench and rachets have been used a few times however its used almost every time i wash the car after drying to get the water out of the crevices.
looking at using it as a hoover as well now for cleaning in the awkward areas where a normal hoover head cant get into