Workups are typically a shorter period of time as it applies to anyone waiting off the ship.
A workup is prepping for deployment. Underway is when they are headed out on deployment. As to time frames, it can depend on the ship or type of ship.
An Underway is not always when they are headed out for deployment. My son's ship went on two Underways before they deployed. Both Underways were for about a month and occurred at 4 months prior and 2 months prior to deployment.
it's all the same…
Workups are for training cycle to prepare for a deployment. Underways are the same thing…they don't go for no reason, its always train, train, train.