I've always heard 16 hours and the first thing I saw when I searched earlier said 16, but I just got to thinking that on the longest summer days here we have sunlight from about 6:00 AM to 10 PM, so that's barely 16 hours on our longest days. So I just looked some more, and the recommended is "from 14-16," with 16 being the max so they have 8 hours to sleep and recuperate. Too much light and they're going to be exhausted.
If you are hitting 14 hours, I'd say that would be fine. The days here in TN now are about 7 AM - 9 PM, so we're getting 14 natural hours.