My last facility and current facility use the air knives by Dyson. Lovely hand dryers, but they must be maintained and the filters changed regularly.
In my last facility we did monthly air quality testing at each hand dryer. Compared it to the air quality in our clean room. If the levels of the hand dryers were worse than our clean room we would change the filter. I wasn't there long enough to establish a baseline frequency in changing the filter at the air dryer, but I did notice a trend in that it depended on how often the air dryer was used.
The real sucker about the Dysons is the pre-filter and HEPA filter are one unit so you have to change an $80 HEPA filter if you want to change the pre-filter. Well, we worked around that in getting our own filter media and cutting it to size. So, we ended up changing out pre-filters as needed. The HEPA filter was an annual change regardless. Changing pre-filters was sufficient enough to bring down the air micro counts. IT also helps to lengthen the life of the HEPA filter.
At my current company I can't tell you the last time the HEPA or pre-filter have been changed....it is on the bottom of my to do list to be honest. We have bigger fish to fry at the moment, and it takes a strong culture to stay on top of that stuff, which we don't have.