Re: How do I use Chlorine in my Spa (or pool)?
For a whole day, you might need to do a decontamination which would require a drain/refill after the treatment (either Spa System Flush or superchlorination) in order to remove biofilms. Bacteria double in population in 15-60 minutes under ideal conditions so while well-established biofilms that are significantly resistant to chlorine might not be formed in a few hours, a whole day (24 hours) could be a problem. Even 8 hours could theoretically be a problem though in practice I think regular shock levels would be sufficient in that situation.
It's a statistical thing depending on nutrients and other conditions. I've seen reports of biofilm within a day or two. Not every time, though. I also assume you don't have an ozonator. Though it doesn't provide a consistent residual, there is usually a little ozone getting into the spa water so takes the edge off of growth, but not enough to completely prevent problems.
You could just see if you've got unusual chlorine demand that is higher than normal. That's the primary indicator for biofilms. Slimy surfaces would be another, but biofilms could be in your pipes or filter so chlorine demand is a more clear indicator, at least when there is a lot of biofilm.