20 minutes seems like a good start.I’m a new chicken mama too, my guys are 4.5 weeks old. They have has a few day trips out. I started at 3 weeks at about 20 minutes and increased it from there. They stay out for little over an hour now.
The first couple times they were terrified and just huddled together, didn’t move much and peeped loudly in protest! Now they are running (as much as they can in their small pen) stretching, grazing and dust bathing in the dirt. So don’t be alarmed if they aren’t into it on the first couple field trips.