Learn Spanish (Latin America): Mientras más tiempo te dejas la mascarilla, más nutres el pelo. - The longer you leave the mask on, the more you nourish your hair.

English uses 'the [comparative]... the [comparative]' (the longer... the more...). Spanish doesn't use definite articles for this. Instead, it uses phrases like mientras más or cuanto más. Here, 'the longer' is expressed as mientras más tiempo (literally 'while more time').

Could I use cuanto más instead of mientras más in this sentence?

Yes, cuanto más tiempo works perfectly here and means the exact same thing. In Latin America, mientras más is extremely common in spoken Spanish, while cuanto más can sometimes sound slightly more formal, but both are completely interchangeable in this grammar structure.

Why is there a te in te dejas? Can I just say dejas?

The te indicates that you are leaving the mask on yourself. The verb dejarse is used when you leave something on your body or let something grow (like a beard). If you just said dejas la mascarilla, it would mean leaving the mask behind somewhere, like on a bathroom counter.

Why is it el pelo instead of tu pelo if the English translation is 'your hair'?