certificates are generated with validity of two years. Googling says the maximum validity is 13 months. Does it mean one needs to renew this process every thirteen months? If so, I would add it to the page. I am just not sure if that is the case. I followed this guide some time ago, then the warnigns started to appear again - I assume it was because the certificates stopped being valid?
The certificates can be as long as you like, after all you specify it upon creation.
The limitation found by googling is more like an agreement if you prefer and affects the public certificates, not the private ones created by self signed CAs. Same as a car can be as fast as the manufacturer makes it, but manufacturers agree to set a limiter at 250km/h.
The letsencrypt certificates have a 3 month validity.