The two main factors influencing this are their chemistry and environmental conditions. Some are marketed as only needing to be applied once-a-season and claim to last up to three months. The issue that we have with this is that you don't know when they stop working so it can lead to periods when effectively no wetting agent is present. If a soil wetting agent lasts a month and you apply on a regular basis then you avoid these issues developing. Always ask for independent data to support any claims made rather than relying on a glossy brochure.