I'm the only one who uses the tub and only twice a week at the most, usually once a week. I'm in it for about 50 minutes at a time.
What usually happens is at about 6 weeks my tub starts foaming and it'll clear once or twice but then it becomes time to dump and refill. With the usage that I give it it should last longer but it doesn't.
Originally I blamed it on my kids bathing suits - washed of course and thinking it was the detergents and spray and wash.
One of my problems is I tried too many things to clear it up ... dichlor, MPS, enzymes and clarifier ... too much crap IMO.
This past water change I use only dichlor mostly, MPS once in a while and Sea Klear clarifier on a weekly basis; nothing else besides baking soda to raise PH and Alk.
I did try N2 but it didn't do anything for me and every so often my water get's chlorine lock. I can go 2 days with the chlorine not disappearing. I had chlorine lock just before this cloudiness episode.
As far as water changing, there is a formula:
# of days = (1/3 volume)/ (# of daily bathers)
so a 300 gal tub used by 2 people would be:
(300/3)/2 = 100/2 = 50 days
Water here is cheap and it really is a great chemical to put in the tub!