How long do you have to wait before adding another layer of product on your face?

Most aestheticians (skin specialists) I know recommend anywhere from 5-10 minutes between products. Keep in mind for every layer you add, you are somewhat diluting it and the product underneath, no matter what. Generally speaking, if you are applying more than three layers you need to rethink your skin regimen. If you are spending a lot of money on OTC or products like Lancome/Estee Lauder, etc. you are better off going to a dermatologist and purchasing skincare lines they carry, which are more concentrated and have scientific studies to back up their efficacy - i.e. Kinerase, MD Forte, SkinCeuticals, etc. If you are going to end up spending the same amount either way, you may as well get your money's worth!