My first backup took about 5 hours total for approximately 550 MB of data backed up to iCloud. I did this on launch day immediately after installing IOS 5. Which BTW went off flawlessly on my iPad 2. I waited until around 1230 PM Central time and had no issues at all with the entire process.
When the backup said it would take 5 hours I put the ipad on it's charger in the bedroom and let it do it's thing.