If your plan is to provide maximum HTTPS coverage, then duration select (1 day - 39 months) should be available during certificate generation.
We would love to use LetsEncrypt in our projects, but many of them run on shared hostings, where there is no possibility to automate generation, and default (unchangable) 90 days expiration period is just way too short.
This leaves us with two options: buy a proper cert, or serve by HTTP. Guess, which one was selected by our client? Yep, the free one - do not use a certificate.