The ACME specification says that authorizations expire, e.g. in sections 7.1.4 and 7.1.6.
ACME doesn’t mandate when they expire.
(The Baseline Requirements all CAs in the web PKI must follow do limit how long CAs can reuse validation information.)
“30 days” is a configuration detail of Let’s Encrypt. It may change in the future.
An ACME client will always know the date and time things expire, but it can only deduce the rules by which the CA might have chosen said date and time.
Edit: An ACME client should always assume it may have to validate, and proceed how the server wants it to.