A DOW filmtec 75 GPD membrane (that is what you purchased isn't it?) has a 98% rejection rejection ratio. I would replace it when it drops below 92% or so, depending on the incoming water TDS.
If you incoming TDS is 150, then you can go a lot longer, as the DI does not have to work very hard. If your incoming TDS is 400, then you need to pay close attention the the membrane life.
400 TDS input:
At 98% rejection your RO TDS is about 8, at 90% rejection it is 40.
200 TDS input:
at 98% rejection RO TDS is about 4, at 90% rejection it is 20.
You can see that at 4 or 8 TDS things are not that much different, yes the RESIN will still have to do DOUBLE the work at the lower TDS, but the "TIME" involved is long. At 20 TDS life will be somewhat short for the resin and it is likely less costly just to replace the MEMBRANE to reduce resin usage. However, at 40 TDS the resin is going to get chewed up in no time.
I hope this give you an idea. There is no "real answer" without doing a lot of math to determine the optimum replacement window based on RESIN cost, INPUT TDS, and MEMBRANE cost.
Bean