well, DV avi's only the intermediate stage. You'll do all your editing, in the DV avi format, then you encode to a format DVD players will play, usually mpeg-2. The mpeg-2 will be WAY smaller than the DV avi files.
as for how long you'll fit on a dvd, it depends on a lot of things. Audio bitrate will eat into the remaining space for video. The mpeg encoder you use will make a difference, as some are better than others, quality-wise, so better ones will give the same visual quality in a lower bitrate. You really want to do VBR, not CBR as well. The video content makes a difference. Is it grainy? High-motion?
AMD Athlon 1700 (@2200mhz from 1466 sod the noise!) - 768mb pc3200 - 200GB Maxtor - Pioneer 106 - GF4 Ti4200 - twinhan/visionplus pci dvb-t - random pci analogue tv tuner - A7v600-x (to replace the a7n8x-d that died...) - random 17\" CRT
Sony STR-DE425 & Eltax c205s;
xBox w/20GB HDD- executer 2.3b;
xBox - standard;
Konica/Minolta dimage z10 w/2x512mb SD;
A silly amount of Cat-5 and usb cables