My RO filter has never been replaced because I don't have one. Unfortunately, I have to do it the old school way. Drive to my lfs which lucky for me is WWC and lug 5 gal jugs back to my house an hour away. I am trusting that their water quality (fresh and salt are ok). Maybe that's my problem.
May be worth buying a $15 handheld TDS meter to check their water and not take their word for it. In my current place I don’t really have a spot for a perm ro/di install so I was buying “fresh” water from a LFS. Few weeks ago I decided to buy one and check the water(visual tank reasons) which read 338 TDS! I thought it was off. Purchased a filter - out of the membrane it read below 10 and 0 after di with both an inline TDS meter and the handheld meter. Pretty much same number for source water also. I had some trust in this store also, not anymore.
This is the one I purchased and seems to work fine for me.
Digital TDS-Meter, Accurate and Reliable, HoneForest TDS, EC & Temp Meter 3 in 1, 0-9990ppm, Ideal Water-Tester-PPM-Meter(Green)