Hi there, Nick here from perceptivemind.com.au. It's good that you already have the design prepared, that's the stage that usually takes up the most time. Being a stand alone app, I reckon it can be finished within two weeks, but also I'd like to know all of its requirements and have a look at the design before I decide for sure. In perceptive mind we never missed a deadline and we never like to over promise, so if we can not deliver it in time I will be sure to let you know from the start.
Feel free to contact me at anytime,
Regards,
Nick.