The TDS level should be under 2000 ppm. When it gets higher than this it can cause skin and eye irritation and metal corrosion of your pool equipment like ladders or hand rails. Cloudy water and scaly deposits also occur more easily when the TDS levels are high. Just like TDS, Calcium hardness is another measurement I like to test once a year, to make sure it does not exceed 1000 ppm. In other states, calcium hardness is considered high at 400 ppm, but in Arizona, water is very hard out of the tap and can be over 300 on a newly filled pool. I personally let it go to 1000 here before I contact a customer about it. Like TDS, high calcium hardness can cause issues with your pool. Cloudy water, calcification of filter cartridges (calcium build up) reducing water flow, and scale build up on tile or metal parts like ladders or hand rails.
When one, or both of these measure high, it is time to change the pool water. It is ideal to swap out the pool water every 3-5 years, but I do test for TDS and calcium hardness every fall.