The answer to how often you should apply a sheet mask strongly depends on your skin type. For some, you can apply it once per day to help your skin, however if you suffer from oily skin then once a week might actually be the ideal amount of time. This is the same if you suffer from acne, then once per week should be the ideal length of time.
Some people like to apply two masks per day, but beyond the hefty cost, we generally don’t recommend this. This will offer diminishing returns, while it could also irritate the skin if applied too often, a sheet mask should really be applied once per day as part of your skincare routine. The only other consideration here is that ‘once per day’ is fine as long as it isn’t a glycolic acid containing sheet mask, as these can be quite intense and over-usage could cause irritation.
Just make sure to get the sheet mask applied in the right order, so first cleanse the skin, use a toner and then apply (potentially with a serum if you have one available).