I'm a fairly new spa owner (approximately two months).
The commonly held rule for spas and hot tubs seems to be to change the water every three months (or more often, depending on who you ask). When I asked my dealer the reason for this I was given a nonsensical answer, so I googled it for myself. Multiple sources have indicated that the reason for this is that the TDS level rises over time. Eventually it gets too high and chlorine is no longer effective; thus, the water must be replaced.
This answer made sense and I accepted it for the past month, but I was doing some additional reading on the forums and there were multiple comments that TDS seldom, if ever, contributes to problems with water chemistry. The most common reason necessitating changing the water is actually a buildup of CYA (and less commonly, calcium). Thus the dichlor-then-bleach method of sanitation.
This also makes sense and several reputable sources seem to concur with the claim, at least when it comes to pools. So now I'm back to wondering, why does spa water need to be changed every three months? Most of the TDS is sodium chloride anyway, which is intentionally added to SWCG pools and spas. If the water is properly maintained, balanced, and sanitized and the CYA levels don't rise above 20-50 ppm, and the TDS level isn't actually an issue (by itself), what is the reason for changing the water?