Question about Upgrading

Hi! I don't see a help section on these forums, and the section for upgrading under the F.A.Q. is "under construction". I donated once forever ago (like 2 or 3 years ago) to get alpha access to the site. And I am debating upgrading again ^^

I was wondering how long do upgrades last? A month? a Year? Forever?

Thanks in advance for any replies!