I can't tell the exact value, but i've recorded some videos of games recently, and i had max 300mb on a 20 second video at 720p at 30fps, so you might be getting 900mb or more if the ratio is equal, on a 30fps.
the difference will be the frames, for example you can tell the framing differente from 30 to 60fps, 30 is slower but also requires less hardware, 60fps is higher, faster and prettier to see but can slow performance while recording.