I would like to upgrade our 2007 Exchange server from the initial release (RTM) to SP1, or if it makes sense to a later SP. I would appreciate an order of magnitude kind of estimate, is it an hour, 6 hours, a weekend?
It's running on a Dell Server PE 2950, with a Xeon 5130 CPU (2 GHz) with 4 GB of RAM. Windows 2003 SP2 x64. We have about a TB of disk running with Dell RAID. Unfortunately, the C drive only has about 5 GB free; it was set up by a consultant years ago as only a 20 GB drive. The data stores for Exchange are around 600 GB on a separate D drive.
Applying a Service Pack to an Exchange Server should not take more than 2 hours time on reasonable hardware. Make sure you have a current backup before you start! I'd also recommend to apply the latest Service Pack (SP3, I think), as well as the latest Update Rollup.
See the Release Notes for Exchange Server 2007 SP3 for more information on compatibility and so on.