All mine seem to have 4.7 Gb of space on them.
This does not equate to a particular length of time on my Sony DVD recorder. This is because I can adjust the legth of recording by reducing picture quality.
Anything we want to watch on a large screen I record at about 2 hours length. Anything I plan to watch back on my little portable DVD I record at 4 hours, as I cannot see the lower quality on the small screen.
I don't know if all recorders have this adjustment possible.
If you buy some make sure you get the right sort, either DVD+RW or DVD-RW, depending on the recorder.