How long can I record on my GoPro?

I'm recording my school play tomorrow and the first act is about an hour and the second is about 40 minutes so I get a break in between. Will my GoPro (First Generation) be able to record the whole thing? My Nikon stops every 20 minutes. I have a 16GB SD card so that should cover it.

Yes, you should be able to cover around 2 hours with 16GB as long as you have the 1080p GoPro, if you have the Ultra HD, it will require more space, just make sure you clear off any unnecessary videos from your SD card first just in case.

Your Nikon only stops because it is NOT a video camera, nor is it designed as one. Clip length is limited by the maximum file size allowed by the memory card's file system. The GoPro is a video camera and designed for video, therefore doesn't have that limitation.