Five to ten minutes a few times a week, and I've not had a back problem in years.
It's a simple and easy way to keep a healthy baseline. Good for all sorts of physical and mental issues. I feel like shit if I've not done them for a while.
It helps, no doubt.
But it's not bullet proof either. A buddy of mine, very active guy, runner, cyclist, rower, tennis, swimmer, works out 10x a week, hikes 2 weeks in the rockies every summer. If anybody knows about warmup and stretching and all that, it's him. Has been a programmer for 20 years. Herniated disc took him out a year ago, still recovering.
What I'm saying is that stretching is going to give you a very good defence against back problems. Been totally life-changing for me.