I've found that a similar principle applies to my helmet camera. When I record in HD, it uses roughly 1 GB every 15 minutes (i.e. 4 GB/hour). The camera came with a 2 GB memory card, but that's too small for my purposes: I often cycle for more than 30 minutes in a single journey. I can swap memory cards while I'm out and about, but I need to plug the camera into a computer to delete files. So, I bought 3 x 16 GB cards; they should handle 12 hours of cycling, and I wouldn't want to do more than that in a single day. (During my LEJOG attempt, my longest day involved about 11 hours of cycling.)
My basic policy is that I use the camera on every journey; after all, I don't know in advance what will happen. However, if it's all routine then I'll delete the video afterwards. I only keep videos where something significant has happened: this could be good (e.g. scenic views) or bad (e.g. a collision). Even when I do need to keep the video, I probably only need a few minutes' excerpt rather than the whole thing.
It's important to be diligent about dealing with these video files. If I do it right away, while the trip is fresh in my mind, it's easy to know whether I can delete it. If I want to keep it, I should make a note, e.g. "saw unicycle about 10 mins in". Otherwise, these things quickly fill up the memory card. I can move them to my hard drive, but that only delays the inevitable. I now have a collection of videos that I need to sort through to free up space, which is a tedious job.
So, if you get a helmet camera, learn from my example: stay on top of the data before you drown in it.