I think We should Provide particular time according to the Contest Prize.
If anyone started $99 logo design contest, so 2-4 days enough for designing for $99, but 14 days for $99 is not Fair not even for contest holder. Nobody attract to a long time contest with minimum price.
For $150 one week is enough (submitting entries)
For $300 you can give more than 7 days or 14 days.
What you all think ??