Thanks for the input everyone. Mine goes weeks without running as well. I do run it up for a minute or two every couple days to keep things wet. I don't know if it matters, just thought it made sense for the RO membrane anyway. Good to know that the TDS output spike is common on startup.
I just checked the TDS on the feed and it went up to 418 (salt pellet softener). The output side dropped from 25 to 1 after about a minute of running.
This membrane looks like the same membrane used in other systems costing 3-5 times as much but yeah, $30 seems sketchy. The output TDS does get down to zero though, so it must be working? Also, I'm not sure what the TDS was when I pulled that first sample. It could have been on startup when output TDS was in the 25ppm range.