ive had mine in maybe 2 months with no service yet, I have the spectrapure 25gpd unit and have upgraded(waiting on shipment) to a 100gpd unit.
I use RO
only not ro
the first unit is better due to it having a silica neutralizer thats the extra cartridge you see.
I think most reefers use the RO
unit. 90gpd is a good unit. usually 3 gallons of waste to 1 gallon of RO
I am assuming filter changes depend on the quality of the water you are conditioning. heavier TDS
water supplies will more then likely go thru filters more then a light TDS