Utilities generally use what is called spinning reserve. That is a plant, or a few plants, that are sitting at idle power with the turbine generator up to speed but may or may not be connected to the grid. The old coal and oil plants were good at this and could quickly meet peak demand. Gas is much better at it though. Nuclear is always base loaded as it's usually the cheapest kilowatt maker in the system. It's never taken me 72 hours to start the nuclear plant I worked at except after an outage where a valve lineup check of the entire plant is performed and mountains of paper work to do.
Your practical knowledge obviously trumps my second hand one. The 72 hour figure came from a guy I knew that worked on the CL&P nukes in Pickering. We talked about it in the context of the cascading failure that caused the widespread grid outage, he said that 'once the nukes shut down we will be down for at least three days because that's how long it will take to bring them back up'.
