When you use project templates, you can adjust your stage dates in the work breakdown. When this template is applied to a new project, the stage dates will proportionally adjust to the project duration.
Clicking on the pencil icon in the WBS and going to view details will show the Stage details page. Refer to stage details for a full blown explanation.
In templates, when you set the start and end date for all the stages, this will scale according to the project duration. What we call proportional logic only works if you set dates for all the stages.
EXAMPLE: My project template only has two stages, proposal development and concept design. In the template, I set the Proposal development stage to last five days, and 10 days for concept design. In between the two dates, I leave a gap of five days for client approval. This means, that the project goes for 20 days. Now, if I were to create a project that goes for 40 days, and apply the same template to this. You will see the proposal development stage will scale to 10 days, and the concept design stage will scale to 20 days. And a gap of 10 days for client approval.