The most I could find is that the Electronic Communications Privacy Act allows for warrants to be issued for emails less than 180 days old. I’ve found vague references and snippets from articles no longer available that seem to claim some acts that have passed since then allow for simple subpoenas instead of full on warrants for said emails, but 180 days is the only threshold I’ve found and again, it’s for less than 180 days that’s at danger.