The reason why you can't find any good data is because it is not a firm number. The range of file size can vary widely as the jpeg compression algorythm can compact some photographs more than others based on the complexity of the image. I just did a quick test, pointing the camera in every direction and across just 15 shots the size variation was from 3.30 to 4.41 MB per photo based on the 12Mp wide mode. Another grouping of 7Mp shots (both wide and medium) ran from 1.90 to 2.70 MB and the final grouping of 5Mp medium ran from 1.55 to 2.17 MB. Photos with a lot of black in them will be even less and photos of scenes like stadium crowds where every adjacent pixel may be different may be larger.

So the best answer is you will know when you are done. And BTW, don't expect too much accuracy from GoPro's internal timer either. Of the two series that I have run previously on my Hero3 Black, the 5 second one (12MP wide) averaged 5.20 seconds per shot and the 2 second one (7Mp medium) averaged 1.85. The numbers get even more interesting when you break this down into groupings based on where they fell in the battery usage.