Depending on the size of your forum, and how fluent you are with moving files around and testing that everything works as you'd expect it to work after your upgrade, it could be as little as 5 minutes.
Generally with this software on standard upgrades from one compatible version to another, I tell my users in a forum post, "the forums may be down for up to 1 hour during a forum software version upgrade and planned maintenance from XX to YY Pacific Time (PT)."
This also works well -
The forums are currently closed while we perform some brief maintenance. We will be back in just a moment.
