Major release upgrades occur every six months, with Long Term Support versions coming out every two years. Routine security and other updates run whenever needed, often daily.

How often should I run apt-get upgrade?

I would run apt-get update; apt-get upgrade at least weekly in order to get any security patches. You should get little no upgrades on 14.04 that aren’t security related at this point if you only have the default repos setup. I wouldn’t bother setting up a cron job; just run the commands once every few days.

How often does Ubuntu update?

Every six months between LTS versions, Canonical publishes an interim release of Ubuntu, with 20.10 being the latest example.