Dispensers also have settable vanes. In my pool 10 years ago when I used Trichlor before I new better, the 3" 8-ounce tablets needed replenishing after around 5 days. They wouldn't be gone by then, but would be down small enough that they wouldn't put out enough chlorine fast enough. However, as Jason says, it really depends on a lot of factors. It's probably the case that for most dispensers it's roughly somewhere between 3 and 7 days. You generally can't go much longer than a week with a dispenser. Adding more tablets just has greater output, unless some are out of the water and drop down as others get used up. Others should chime in since perhaps there are dispensers where the tablets dissolve more slowly.