Rob, typically I never hold a stretch for longer than a minute at a time because I’ve heard of research saying that your tendons can get stretched if you hold a stretch past that duration. Not sure if I should listen to it or not, but sometimes a minute isn’t even long enough to help reach that end range! During that minute I will do some PNF or AIS.
I’m always relaxed when I stretch, breathing normally with relaxed face and chest. If one side is tighter I usually stretch that side 2x as much as the less tighter side. As far as the hip flexors go I’ve just worked my way to that end range position and immediately notice that I can deeper in my squats easier. My next goal is to work on my hamstring and glute end range.