If you want to list your item for longer than ten days using Auction format then when you set up the listing activate the option to automatically relist the item if it doesn't sell. That way the item will automatically be relisted if there are no bids the first time, in which case the item will reappear in the search results with the countdown to the end of the listing reset for the same period of time that you set it to run for the first time. If you subsequently receive bids for the item after the advert is reinstated then the listing will not automatically relist afterwards, as the item will no longer be available for sale once the winning bidder has paid you for it.
In the event that you change your mind about enabling the automatic relisting option you can always end the listing manually if you decide that you don't want to relist it again. Alternatively you could advertise the item using the Buy It Now format, in which case if nobody commits to buying it then the item will automatically relist until either somebody buys it or you change your mind and decide to cancel the listing. You don't even have to set the item up to automatically relist if you decide to list it using the Buy It Now format - about a year ago eBay decided to change the rules relating to Buy It Now listings and changed them so that all Buy It Now listings automatically relist until either the item sells or the seller withdraws the listing. Therefore you can't set a Buy It Now listing up for a period of thirty days like you used to be able to do, so if you don't want your items to keep on relisting indefinitely until somebody buys them or you decide to end the listings then the best option is to stick to the Auction format for listing your items.
With regards to your comment "There are occasions when I would like to list for 8 or 9 days..." if you only want to list the item for eight or nine days then set the listing up using Auction format and select the ten day option. If you only want the listing to run for eight or nine days then end the listing early on day eight or day nine, selecting the option to sell to the highest bidder. This is the only way that I can think of that would enable you to have an Auction listing live for longer than seven days, but less than ten days.
