How do I change the ebay listing duration in my profile to a default duration

[mustangabby]

I have tried to find out to revise a listing from a 30-day auction to 7-day auction, but it does not give me that option when I list or when I try to revise.  I dont know if this is because I am doing fixed-price and Best Offer listings, but I want to change them to 7 day duration.  Have been unable to find information on how to do this, or in my seller preferences profile.