It would probably be a few hours tbh.

The grid has to be balanced. If your town starts using more power a power plant somewhere in the network has to spool up more generators or increase their speed.

If a power plant suddenly goes offline the power plant has to call the regional area offices and tell them so they can balance the load. I've been in a plant where this was happening a lot. Even if this wasn't automated it could easily fail somewhere. The electric grid is one of the most complicated pieces of infrastructure in the country.