Ours is a 100gpd (ro man) and to start with it we got about 018 tds I think and the tap water is between 250 and 350.
After reading through some posts we added an extra filter to the RO system (I'll have to find out what filter) and an extra resin filter. We now get 001 to 004 tds.
To begin with the resin lasted maybe a week at most but after we added the extra filter it now lasts about a month to 6 weeks and the things is running every day from about 9am to about 6pm.
Cheers