In theory 14 days: "You can choose a winner at any time when the contest is active and within 14 days after the contest ends. If you need help, create a poll and get your friends to vote on up to 8 designs! Note: If your contest was guaranteed the prize money will be automatically shared between freelancers if you do not award the prize within 14 days. "
I am not in favor of total ripoff but can tolerate some elements from other designs..i do look at them myself and there is nothing wrong but one should not take them for granted and claim it as theirs.
Don't feel bad for that, you never know what is the client's choice. Their choices are not the same. Some of them chooses the good one, and ............... you know what i mean. so nothing to feel bad. Its totally their own taste. :)