How they do backups is defined by the system used and what they need backed up... you are asking us "how long is a bit of string" mate
what server (08, 08r2, 12, 12r2)... now days we tend to assume Hyper-v (=V12r2) so that allows us to copy the server and then take the old one down or transfer it to another cluster if turning off isn't needed.
what do you need backed up... just the client data stored on it, a copy of the basic server before adding a new task like WSUS so you can roll it back in case you bugger something up or a total working (i.e, rolls installed and windows activated) copy.
Agreed, the solution is going to be wildly different between enterprises. It's going to come down to how much time (if any) can you afford to be down and what kind of a budget you have available. Many bigger enterprises are going to have SANs storage clusters, multiple active active data centers, some kind of redundant virtual infrastructure etc.