MPS is a decent oxidizer, but it doesn't stick around very long, not usually more than a day. The decomposition rate of MPS at spa temperatures is about 12% per hour so is 90% decomposed after 18 hours (95% decomposed after 24 hours). Chlorine can last quite a bit longer, dropping about 25% per day when the tub is not in use (if you have an ozonator or are have accumulated stuff to oxidize in the spa, then the chlorine drop can be 50% or more per day).
Bather waste from sweat and urine will consume a lot of oxidizer. At hot spa temperatures (100F-104F), one person-hour of soaking takes around 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of MPS to oxidize this bather waste. Many people simply do not use enough oxidizer in their spa. The typical 2 teaspoons of Dichlor is good for one person soaking around 30 minutes or two people for 15 minutes.
So if you are going to be using primarily MPS (only if you have Nature2 or another silver ion system), then you will need to add it every day if you are using the tub that frequently. If you are using primarily chlorine, then you can go a few days without adding chlorine if you aren't using the tub (as Vinny says, you don't want to go too long before you add chlorine) but whenever you use the tub you'll need to add chlorine either before or after each use (depending on whether you want to soak in higher chlorine levels or not).
Richard
