Joshua Naterman said:It doesn't have to be painful, and your body adapts pretty quickly. More than anything, it's boring. However, it does work!
I would only focus on one stretch at a time for long sessions, should you decide to adopt them as a training protocol, and spend regular time on everything else. Then, as significant progress is made over a few months, switch focus to another stretch and spend time there.
You have to be very careful with bridge and shoulder stretching, you can easily compress nerves and cause damage, so shorter duration stretches may be better for those. I know I have had issues with this, but I also have full cervical ribs on C6 and C7 so I can't exactly be used as any kind of predictive example for the general population. Nonetheless, I would approach with caution for shoulder opening stretches.
So the main idea is to work specifically for a certain stretch for example pike or bridge and hold it for 20-30 minutes one your hard stretching days. While keeping everything else held for around 3 minutes. And as for the rest of the week, stretching 5-10 minutes on one your deficits while keeping everything else low in duration for maintenance?