grammatical number - What is the plural of "hair" when it refers to the hair of multiple people?

I understand that "hairs" is plural when talking about multiple hairs, as in "my hairs have split ends", and that "hair" is singular when talking about ones collection of hairs, such as "my hair is blue". However, I'd like to know what the plural form is when I am talking about the collections of hairs of multiple people.

"We must wait until our __ dry"

Furthermore, if the answer is "hair" which is typically singular, then would it be "We must wait until our hair dry" or "We must wait until our hair dries"?

Answer

Hair can be either hair or hairs when pluralised, but the hair(s) of a group should be hair. I have three grey hairs, but we all have brown hair.

My hair, your hair, our hair.

"We must wait until our hair dries"

Hair is usually hairs when it's countable, and hair when uncountable. As an uncountable, it becomes a mass noun.