10 days is a lot and could be a problem.

Payment not guaranteed is a problem.

About the 10 days deadline,

first of all, a shorter deadline will get the best designers working for you as soon as possible, a long deadline would just make them postpone again and again until they forget about it.

Second, designs will come in a lot quicker when the deadline is closing in.

And third, a week from now, we would hardly find your contest, it would be on the 10th page or something like that, that can't be good.

Thank you for the feedback and submitting your designs. Unfortunately I don''t think I can change the deadline to be shorter, only longer. 

You're welcome, it's in everyone's best interest after all.

About making the deadline shorter, you can, it's been done before by many others. However, i have no idea how as i've never held a contest.