I would hesitate to suggest a "finishing time" for your first event, but rather encourage you to prepare yourself to be able to go the distance comfortably and with purpose. Besides, it's still very early in your training to be able to predict what your 5K pace is likely to be. You'll get a better feel for that as you put more miles on and get closer to actually running 3 miles. But don't fret over it.
The key thing, I believe, is to first be able to get to the finish line. How long it takes you is secondary. As I started running (a mere 5 years ago), my mantra was "cross the finish line, preferably upright, and preferably running!". The even cooler part was when, with say a half mile to go to the finish and realizing that I had a lot more endurance than I thought I would have, would be to boost up my pace and actually pass people on my way to the finish. That is a huge confidence builder!
Good luck and have fun with it!
-bill