You would manage this feature within any fee based price plan within the COST SPECIFICS admin tool page for that feature. Here is the full click path:
PRICING > PRICE PLANS HOME > [EDIT] (next to price plan to edit > COST SPECIFICS
You would look for the following section in the COST SPECIFICS admin tool page:
To select "charge by duration" for users attached to that price plan:
click this radio button and click save
then click the Edit Listing "Duration" Choices link to edit the duration choices and their associated price the clients attached to this price plan will see.
After clicking the edit listing duration choices link you would see a page like:
You would do these to insert a duration choice and delete one:
Insert the displayed text for that choice that will display on the client side within the duration choice dropdown for users in this price plan. Note there is no language specific settings so "cross-language text" should be used or text descriptions in multiple languages within that text.
Insert the numerical value noting the number of days for that duration choice. Note that you can user 1.5 to represent 1 and a half days.
Insert the price the client will be charged when placing a new listing
Insert the price the client will be charged if renewing a listing
Once all four of the previous fields have been filled in click the save button to save that choice.
To remove any duration option click the delete button next to it.
The above points you to the price plan wide settings. You can also set this feature up on a category by category basis within each fee based price plan as linked here:
PRICING > PRICE PLANS HOME > [EDIT] (next to price plan to edit > CATEGORY SPECIFIC COSTS > (SELECT CATEGORY) (see setting up category specific pricing to set)