Q: How long should Yosemite update take?

An update or upgrade? Read Software update, upgrade--what's the difference? - Apple Support.

There are two parts to upgrading an earlier version of OS X to Yosemite: downloading it and installing it. The time required to download it is a function of your ISP's speed as well as your wireless environment, assuming you're using wireless. If you have a reasonably fast connection, that will be about an hour. It can take a few times that much if your connection is particularly slow.

Installing Yosemite will take 30 to 40 minutes or so.

Incremental updates occur considerably faster than that, but they also require the same two parts: downloading and installing.