How often does one suppose to change the water filter in the Hiker Pro? I been cleaning my filter out at home and noticed th filter is blackish on the bottom half and still clean looking on the top after rinsing it a few times.

I have probally run about 10 gallons of water thru the system since I bought it a few years ago. Yes, I clean the filter and dry it every time I am home from a trip. Just curious,since I seem not to be able to find my directions