Time to image large drives (80 gig+)?

Just wondering what you think the average time would be to do a full backup (and following that, incremental backups) of a large partition such as 80 gigs plus. I am thinking of backing up my multimedia files every week and am curious as to the length of time it would take to create the first image of approximately 80 gigs (onto a usb 2.0 external hard drive) and subsequent incremental backsup of maybe 100 megs at a time?
This would be using none (or minimal) compression since everything is packed pretty tight already (mp3s, etc).
Thanks for any guesstimates!