You will need the Full Write Service Plans permission to perform this action.
Navigate to the plan you want to copy and choose "Copy plan" from the Actions menu in the upper right.
A pop up will appear where you can control how the plan is copied. The options here are essentially the same as when you create new service plans. In this case however, you are basically using the existing plan as a template for the new plans. You can copy this plan into one or more new plans in bulk.
The "Preview" section will list all the plans that will be created along with the dates and times. The "Options" section at the bottom lets you control exactly what is copied over.
Once finished, click on "Copy" to perform the copy. You will then be redirected to the earliest plan created (if many).