When do you justify a 10 day auction?

Obviously the cost of .40 might play a factor if you're listing 100's of cards that will sell for less then $5, but what about Mantle Mays Slemente etc.? Do the extra 3 days get you 1 more new bidder to validate .40? I used to think so, but wondering now?
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That Slemente error card you might want to go 30 days