My experience is with Amphibalanus improvisus, and we used cyprids that had been in the fridge for up to two weeks. But at that time they are much less prone to settle. I usually used my cyprids 1-6 days after the rearing was completed. To collect them little by little and then use them in an experiment is a bit risky, however if you don't have an alternative that's the way to go. Then I would keep the cyprids you collect from day do day separate and at the start of the experiment pool them and make sure you homogenize the batches properly.