Yeah it's typically a 3-4 hour process, minimum here. Often they pull another company out of service to help out with the changeover (if nothing else for the unloading, and reloading a couple thousand feet of 4" and 2.5" hose...)
It's actually easier for them, when an Engine has a relatively minor maintenance issue, that'll pull it out of service for a few hours/half a day, to just pull that company out of service and let the mechanics do whatever, than it is to try and do a changeover unless the truck is going to be out for days...