It depends heavily on the compression used. There is a bitrate setting under the recording setup, that will tell you how much space / unit of time that can be stored.
The OSD cuts recordings at approx. 2GB to be compatible with FAT32 drives.
(Nitpick) I think it is 4 hours (i.e., it is based on recording time, not on file size). This is based on the fact that the default settings result in about 1GB/hour, so 4 hours is 4GB (which is the file size limit on FAT32).
As a side data point, when I knock the quality settings down to about half the defaults (half in the sense of half the file size - which is not exactly equivalent to halving the quality numbers themselves), then I get files that are (exactly) 4 hours in length, but file sizes of about 2GB.
