I think it's 36 months right now, but that's a Baseline Requirements thing, Let's Encrypt can (and does) use shorter periods. 36 months is the upper limit for all CAs. Let's Encrypt currently uses 90 days, and will go to something like 7 days in the near-future[1].

[1]: https://community.letsencrypt.org/t/upcoming-api-changes/179...