So you are doing a daily eating schedule between 9am - 6pm (have to convert for my non-military brain).
There are alot of options such as what is your goals for IF, what are your energy requirements for the day/activities, etc. There are several ways to IF, one can be the daily style, another can be every 3rd day. I would say for maximum health benefits and to make sure you are eating enough, to fast every 3rd day for at least 18hours. Plus to back it up some I would say get your food eating down first, aka eat paleo and get rid of the crap in your diet if any. From there it's something you play with and tweak to meet your performance needs, recovery needs and goals (fat loss, muscle gain, etc).
You could also do the warrior diet style which is more light eating during the day and a larger meal at night (personally I enjoy eating this way anyways as I find better energy and more mental clarity throughout the day). You could eat only meat/veggies throughout the day and then more carbs at night (although I would recommend them in the pwo window for optimal results)
As for fasting times, 15hours is minimal I believe to get any health benefits from fasting...and i am sure there are potentially much greater health benefits to longer fasting of 18-24 hours, but those can not be done daily by most people (18hours can...but you have to be able to take in a ton of food/protein in that small window if you are not wanting muscle loss) Overall, I come to think that for optimal fat loss and health you need a strong metabolism and longer fast periods, for that reason I think every 3rd day is a good start for most people. Daily IF while getting good results for alot of people, if done too long all the time (like 18hrs 7x a week) may lower the metabolism...which is not what you want in a weight loss phase. You can adjust however you see fit, 15hours can probably be done and keep the metabolism strong...but honestly eating from 9am-6pm only is not what I would really call fasting by the health benefits side(I tend to term it at longer periods of 18hours....but that is just me). At that point you probably see the better fat loss and health benefits of just eating Paleo all day long than 15hour fast and not eating 100% clean.
In the end...try something and see how it works. Learn how to adjust your macronutrients like getting enough protein for muscle building, healthy fats for energy, and enough carbs to keep your energy up while still getting fat loss.
I kind of went over some starting ideas on my blog below, and Allen made some good feedback observations in the comments on there as well. Take a look over there too. www.theiflife.com