I change all the filters in the RO
every 6 months. It doesn't take much to clog a 5 or 1 micron hole!...I personally replace the membrane every year as well though others may disagree. The TDS
meters are an ok tool to use to monitor it though output (as you now know) and water quality in your tank are the tell tale signs. It is true output is slower in the winter however, if you house the unit in your house, where we will assume it is heated, then output, at least for mine, is not affected.