how long can 4s take video?

The video length is only limited by the amount of memory you have available on your iPhone. From what I've found, the 1080p video on the 4s uses around 200 MB per minute. Divide the amount of free memory on your phone by 200 MB and you'll have a good rough estimate on how much time (in minutes) you'll have to record.

This website did a great job of breaking it down:

http://whenwillapple.com/blog/2011/10/05/with-the-iphone-4s-improved-camera-and- 1080p-hd-video-should-i-go-32gb-or-64gb/