Certainly give a length of time (an announced length of time), reasonable time, that contest runs. Say, 1 month. NOT "Hey check out this giveaway" and 2 days later entry is closed. NOT GOOD.
Also, simple contribution in the past (no minimum post count) should be sufficient to qualify.
It's obvious that "minimum post count" votes are self-serving those voters: it increases their odds by leaving out the newbies / read-only members, ie, those who are learning the trade...