Weekly. That’s how often I clean our bath toys. They build up grime and mildew if we don’t. Look closely at your toys and evaluate. Try not to puke.
In a clean basin, tub or sink-soak the toys in a mixture of dawn soap and vinegar. Dawn works wonders. I also like to use sterilization tablets. These are leftover from bottle days and help even more. If you don’t have them, no biggie.
After scrubbing and soaking; empty all water from toys and let toys dry.
As a rule I don’t allow squeezable toys. The ones that suck up water and squirt it back out-those harbor mold and cannot really be cleaned. Some people super glue the hole shut on those but I don’t have time for that.