Depends on your bitrate. Here's a calculator for mp3 (interactive, nothing to download), but I'm guessing equivalent bitrate AAC files would be similar in size (I've also read that AAC needs lower bitrates to get similar quality to MP3):
http://www.rvmsounds.com/calculator/
e.g., 1 hour @ 256kbps = 112.5mb
If you're using Apple Lossless, then the bitrate is going to be over 800kbps, so figure an hour would be close to 400mb (unless it's a more space-efficient algorithm I don't know about).