Membranes should be run no less than once a week to avoid biofouling. Remember that all the water in the system beyond the carbon block is not chlorinated. Stagnant water with no disinfectant makes a wonderful environment for microbial fouling.
Suggestion: Remove your membrane from the housing and smell both ends of the membrane - if the membrane is fouled, it will stink in a big way.
Amison - sounds like what you have is an RO system rather than an RODI system, correct? The three stages would be:
sediment filter
carbon block
RO membrane
If that's the case, you'll not see 0 ppm TDS water from the system. You'll need to add a DI stage to get that.
Those 90 gallon per day membranes are 75 gpd membranes where the vendor tells you to run it at a pressure higher than the manufacturer's spec. Given your low pressure and cold water, you'll see much less than 75 gpd. I ran some example situations through the calculator on our website as examples:
40 psi and 65 F water: 46 gpd, or about 2 gph
40 psi and 55 F water: 37 gpd, or about 1.5 gph
40 psi and 50 F water: 34 gpd, or about 1.4 gph
Russ