Its not wise to delay filter changes UNLESS, key word unless, you are using the new 0.2 micron absolute rated prefilter and a 0.5 micron carbon block. Take a look at Spectrapures website and read about the bacteria and virus removal capabilities of this new prefilter. I just bought one for testing myself. I plan to run it as long as I can before seeing chlorine breakthrough at the carbon block to see if I can get the full 20,000 gallons out of the carbon.
With your pressure even if you have low water temps you should still get pretty good production. Take a look at the Buckeye Field Supply Sponsors Forum for Russes output calculator, if you know press and temp it will calculate the output. Good stuff.