You can also use MPEG1 or even half D1 formats to further reduce file size and cram even more on there. You should use AC3 audio, too as it is *much* smaller than .aiff, and also has a lower bitrate. Just remember that your visual quality will also reduce along with the file size when you over-compress or use MPEG1, etc.
If your audio is stereo and encoded at 128Kbps then you'll get quite a few hours of MPEG1 on there. Similar sizes for Half D1 too. If your footage is reasonably still, like a talking head interview, then you can probably encode to MPEG2 at a fairly low rate and get more than a couple of hours on there. You'll need to try the different settings and see which works best for you.
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