A merge from upstream once a day, at the beginning of the day.
I'm working on a DevOps setting, and even though we're a small team, we have about two to three changes going through the pipeline a day.
If you keep your fork too long without syncing, it just get more complicated to merge, and more importantly if you need help from the upstream change author they'll have moved on to another subject and the change won't be as fresh in their mind as if you had merged the day after they pushed it.
MS Dos 5.0 on my first PC was a bit short on features and I had not enough money for Windows 3.1... I heard that American students were using something called Unix and that their was something close available through mail-order CDs. Yggdrasil CDs were cheap too!