Let's Encrypt isn't exactly hiding that it's only practical in the context of automation

From their FAQ (https://letsencrypt.org/docs/faq/):

What is the lifetime for Let’s Encrypt certificates? For how long are they valid?

Our certificates are valid for 90 days. You can read about why here.

There is no way to adjust this, there are no exceptions. We recommend automatically renewing your certificates every 60 days.