Your skin takes around 15-20 minutes to absorb whatever you apply on it. After that time, the product that has not been absorbed will just stay there, a layer on your skin.
So, keeping that face pack on for a longer time won't do anything for your skin.
With creams and moisturisers, you can wash them off after a quarter of an hour to avoid a sticky trap for dust; as well as to give your sunscreen a better hold. If you don't wear sunscreen, you might think of the cream as a protective barrier between the skin and the environment (or the cold), and in that case you can leave it on. Just be sure to cleanse it off when you get home !
This also means that you can technically apply your night cream before bed time and wash it off before hitting the sheets if the guy of your life doesn't care for the taste.
