FF 2x a day is not bad. I only do one session. Rest of the day is marking. Taking the dog out of the field to do FF, in my opinion, is "old school". If you do it right, it's no longer necessary (for MOST, NOT all, dogs) to remove them from the field.
I have four dogs in various stages of FF right now, we just got back from marks and they all did super. It's a great way to maintain attitude. In the field, my standard for delivery is 1-3 steps behind wherever I am in the yard. So I have several dogs in the middle of early ear pinch. In the field, if they drop, I just put it back in, say hold, heel them a bit and run the next mark...
My FF lessons are anywhere from just a few minutes to 20 minutes. Typically lesson is probably 10-15 minutes. I try to do enough in a lesson that I know they understand what they learned last time and I add in the next step. I want to see success in each lesson, in small increments. If I see that success within a few minutes, the lesson is done. If I have a dog I'm fighting with, the lesson will typically go longer...
I like doing one session a day, but there's nothing wrong with two as long as your dog's attitude is good.
-K