I've had one for over 15 years allow to service the first thereafter I did some research and did it myself. I went to look for shops that sold he equipment and replacements. Some machines the filters or outside majority are in the inside. In general, you have a Carbon, Risen, Mircon, then the filter that do the reverse osmosis.
Years ago, I read an article, the government standard said every two months but didn't specify which filter or all need to be replace. If replacing all it makes no sense since the cost exceeds profit. Replacing all cost was 3000 baht, years now I've found shops total filter replacement 1600 baht.
You will find some with stickers at top when they were serviced but that is rear, many machines are just dirty from the outside owners don't take the time to even wipe them down. Thais are funny they think the newer the machine the cleaner the water. I once had to call for service to replace a motor the service guy said your machine is old should replace it once he got it up running commented you water is really cleaned!
In the beginning being the only one in the area my better half was making a killing, 6-7 baht a month. Each month I monitor the filter and examine them then read the more water you sell the more often makes sense but the main filter is to replace the Mircon 50, these are very cheap like 50 baht which today I change each month regardless. The rest based on the volume I sell it has come down to every 6 months.
There is a clue as to when, every machine has a blue vent pipe that water comes out, if filter isn't changed or starts to clog more water will start to run out of this pipe if still not change it will back up and start to leak from the inside. This is a clear sign for me at least to stay away!
Otherwise it is a crab shoot! But they are cheap!