With your setup I would guess is good only for 8x or 12x and may not be able to reach 16x burn speed. Increase your memory a little bit (to 256 or 512MB) and definetely add more HDs.
Normally on my 2Ghz machine I can get 20-25 minutes for DVD Shrink (from Dual Layer to 4.7GB) and burn it around 6-10 minutes depending on burn speed selected. For your 700Mhz Celeron, I would guess you need about 1 hour of Shrink and few minutes for DVD Decrypter to burn.
7+ hours for DVD Shrink is not normal unless he is using a 486DX machine or older.