As mentioned, the time it takes to upgrade is dependent on several factors including the amount of data on your iPad, type of iPad, server load and probably most importantly your broadband connection/speed.
All of the data needed to upgrade is not only downloaded but data is uploaded and also restored. All this data moves between your computer/iPad and Apple's servers. This data transfer is subject to everything that happens on your computer, between your location and the destination as well as the load on Apple's servers. Your ISP's infrastructure will have an impact. The distance between your house and your ISP's head-end factors in. The distance and infrastructure between your ISP and network trunks has something to do with it as well. Even your modem, router and how your computer connects to your network (via Ethernet or wireless) will have an impact. All of that gobbledigook and much more affects the bottom line...your connection. No one's experience is going to be identical.
The "pipe" between your computer and Apple's servers is subject to any number of issues including packet loss, interference, data loads (you're not the only one on the line), etc. The speed of your broadband connection as set by your ISP could be equal to or orders of magnitude faster or slower than someone else's.
For example, I have 25Mbps upload and 25Mbps download broadband speeds on fiber optic service...fairly fast as compared to the U.S. average (about 4 Mbps). My iMac is connected to my network via Ethernet. My iPad has about 55GB's of data on it (although a majority of that is music and videos). It took a little less than two hours for it to be fully updated to iOS 5 between about 10:10 a.m. and noon yesterday. It's taken others less time and yet others quite a bit longer. Again, it's not a level playing field plus Apple's server load is no doubt still very heavy.
So be sure your iPad is fully charged, connections are good and things will work out. Me? I'd probably wait a couple of days to avoid any frustration. The new features in iOS 5 are great, but they aren't earth shaking and as I've said...everyone's devices will continue to work perfectly fine in the meantime.
Hope that helps!