How long does it take a block heater to warm up?

2.5 - 3.5 hours on most v8s. The larger the engine the more time. 1.5 - 2.0 hours on 4 and smaller 6 litre engines. Ie. A 3.3 litre on a Dodge minivan needs approximately 2.5-3 hours. This is approximate, plugging your car overnight is the way to go. If your after saving some energy then 4 hours will cut for most cars.