Depends on how many processes it needs to boot and whether you have to start any of them manually. If the whole lot's auto booting, a couple of minutes. If they're not all autoboot (and considering the amount of custom code that's probably in this machine, quite likely not), couple of hours. That's easy.
Finding out what the hell caused your cluster to suicide in the first place and/or removing the smoking/flaming motherboard with tongs... slightly longer.