I was wondering how much time will we need to move all the files from windows server 2003/exchange 2003 to windows server 2008/exchange 2010. We are planning to upgrade our servers, but if its really a big time consuming task we might just stick with 2003. We haven't done anything similar like this before so I wanted to hear from some ppl who did something similar how much time does it take for them. What do you guys think? Thanks for your opinion.
Migrating Exchange is going to depend on your mailboxes, as @gravyface points out. Since 2010 is 64-bit only, you'll need to migrate as opposed to a straight upgrade.
I recommend reading up on this process to understand what's involved, instead of just asking "how long will it take?", simply because it depends on your present infrastructure.
To upgrade Exchange 2003 to Exchange 2010 will require a new server - you can't do an in place upgrade. You said you're moving to Windows Server 2008 - I would recommend you get the R2 release as that is the most recent supported version.
To migrate to Exchange 2010 requires you to do some preparation work beforehand, but the benefit is that the 2 servers can co-exist while you transition. You can gradually move mailboxes and services to Exchange 2010 with very little disruption at all.
As for how long it takes, that is entirely dependant on the size of your mailboxes. Bear in mind that Exchange 2010 SP1 can only do 2 concurrent mailbox moves at a time. When you move a mailbox it will temporarily disconnect any Outlook or ActiveSync sessions while the mailbox is "in transit". Once the move has completed, the user will be asked to restart Outlook and they can carry on where they left off. We're migrating mailboxes in the evenings so nobody even notices their mailbox has moved servers.
In my experience, a mailbox of about 500MB takes about 5 minutes.