That depends on what you mean by "decommission". Aside from deleting the system in the service marketplace, there's not much you need to do from an SAP point of view. If the server is now completely redundant, you probably want to secure wipe the disks and dispose of it or repurpose it. There's no need to uninstall unless you can't wipe the disks...
Consequently, it is hard to say how long it will take. Since our servers are all virtual and we reuse the storage, decommissioning takes all of 5 minutes...
Steve.