You'd get that by using the lowest possible bitrate in mono. For MP3, that'd be 32kbps. You're interested in the amount of time that would fit in 700MB. That's 179,200 seconds or 2986 minutes and 40 seconds, or 49 hours, 46 minutes, and 40 seconds.

You probably can't actually fit 700MB on a CD, so I'd chop it down a little to make sure you can fit it all there. Even if you call it 24 hours, you'd probably be fine, unless you're a trucker and have a lot of crank.

Just make sure to tell your encoder to do 32kbps CBR in mono. I doubt that your stereo will have a problem with the 32kbps, but it might have a problem with mono. If it can't do mono for some reason, tell your encoder to do joint stereo. You'll probably get nearly as much room as mono. Again, 24 hours should fit and be more than enough.