When we did our contest for our logo and profile banner, we were looking to get a minimum of five entries. For the logo we had enough in the first week, for the banner we extended for a few days until we had enough entries. As it turned out, the winning entry for the banner came during the extension period. But we were clear up front that the contests would extend if there weren't enough entries. (:
I think a big part of success on Steem (and in life) is flexibility. You try something, you evaluate the results, you adjust as needed to keep moving toward your goals.