Which Go-Pro did you buy? This affects what the camera is capable of transferring, so your card has to match at least that. The other things to consider are your settings on the camera that will affect the amount of data generated, such as image size (eg 1024x76

, quality of the HD recording, the frame rate etc.
I have an original Go-Pro HD hero with a scandisk 8GB Class 10 (20Mb/s) and that is fine at 1080p,
1920x1080 default frame rate. YMMV with your unit.
EDIT: Corrected image size used.