Word to denote time to expiration

Solution 1:

Different words can be used for this. In terms of lifespan (itself such a word), longevity is often used: "the length or duration of life." In terms of products, we often use shelf life: "the term or period during which a stored commodity remains effective, useful, or suitable for consumption."

Solution 2:

You could use the word lifetime:

the duration of a thing's existence or usefulness.

"a plan to extend the lifetime of satellites"

"The password has a lifetime of 10 hours."

"The password's lifetime is 10 hours."

"The lifetime of the password is 10 hours."

Solution 3:

The password has a lifespan of 10 hours.

Lifespan — ODO

noun The length of time for which a person or animal lives or a thing functions

From IBM Knowledge Center

"The password lifespan specifies the period of time before the password of a user account expires."