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Judge DredThree days might be a bit short, especially if you start it on a Friday and run it through the weekend.
Seven days seems pretty good. A "search" for a particular item doesn't always bring up everything. Seven days will allow people to do the same search a few times and possibly bring up the item in second or third search. The more exposure time, the better.
Ten days is also good if you're patient. There's a good chance you'll make up the extra dime for selecting this option.