I did a 27-second video yesterday using the iPhone 4S "back camera" and when I pulled it off the phone using iPhoto and the USB cable, it was 84.1 MB. Thus, I'm figuring about 3.11 Megabytes per second. So that's about 5.35 minutes per GB.
I also transfered it via Dropbox. My Dropbox on the iPhone 4S defaulted to a video upload quality of "Medium" which changed the resolution of the video from HD to 568x320, and file size was only a couple MB. When I changed the Dropbox upload quality to "Original" the file that got transfered was 84.1 MB, and it was HD, 1920x1080 pixels. Luckily I was on WiFi and not 3G when I did that!
The Dropbox setting of video "High" gave me a small file and 640x360 pixels.
It appears that the phone's back camera always shoots in HD mode and there's no way to change it, that I can find. I haven't tried the front camera.
Your mileage may vary.