You can play with your Orbeez™ several times before needing to replace them. Like any water-based product, they will need to be replaced with fresh Orbeez™ if you smell or detect any mold growth.
You can keep your Orbeez™ fresh for a longer period of time by always washing your hands and feet before playing with, or placing them in Orbeez™.
Between playing, you should also store your Orbeez™ in a closed container (e.g. Zip-Lock bag, Tupperware, or the container you purchased them in) out of sunlight to keep them fresh for longer.