Just so everyone is on the same page here, the industry formula for when to replace spa water is:
Water Replacement Interval (WRI) in days = (1/3) x (Spa Volume in Gallons) / (# of Bathers)
The soak time that is assumed for each "bather" is probably around 20 minutes, maybe 30 minutes tops. The above formula seems to work reasonably well in spas using Dichlor-only with no ozonator. For spas being maintained using Dichlor-then-bleach, one typically gets twice the length of time between water changes and the water quality is better at the end of that time as well (so one could probably go longer if one wanted to).
As was noted in earlier posts, there is a buildup of unoxidized organics over time. A spa with a functioning ozonator might do better since ozone will oxidize some chemicals that chlorine won't. The ammonia and urea from your sweat and urine should get fully oxidized by chlorine, but other chemicals, especially fats and oils that don't have nitrogen sites, don't get oxidized readily by chlorine.