This is only an option within fee based price plans.

To set listing durations for users in a specific price plan you would "extend" the charge by duration feature of price plan. To set that up you would click edit next to the fee based price plan you wish to set duration choices for as shown in the following screenshot:

PRICING > PRICE PLANS HOME

Then click Cost Specifics:

You would set your price plan to "charge by duration" as shown in the following screenshot:

Then click the Edit Listing "Duration" Choices link. You will see an admin tool like this:

Within that screenshot you'll see:

1. displayed value for each duration
2. numeric value in days for each duration
3. fee charged if placing a new listing. If you are charging nothing for listing placement you would set these all to 0. If you were going to charge a fixed fee for all listing placements you would set all these values to the "charge per listing" you were going to charge. Of course you can also decide to charge differently for each duration if you like.
4. fee charged for listing renewal if allowed….see 4 above for additional comments
5. delete button to remove a specific duration choice
6. displayed value of a new duration
7. numeric value in days for a new duration
8. fee charged if new duration is chosen during the place a listing process
9. fee charged if new duration is chosen during the listing renewal process
10. save button to add the new duration if 6-9 have been filled in

Setting your price plan to charge by duration trumps/overrides any other duration choices set site wide or category specific