Not enough. My goal is twice daily + before and after every ride. I am pretty good about doing the post-ride stretch, and I probably am averaging once a day on the routine one.
But, I would like to offer a caution to those who don't stretch. I have ended up with what may be a chronic problem with my achilles tendon. The original source of the injury was severe cramping in my calf muscles combined with falling center of my foot (due to aging). Had I been better about stretching at the time, I would likely have not damaged my achilles through the calf-cramps. Lesson learned. And stretching is now one of the best things that I can do to stay pain free.
Stretching will also make you more comfortable on the bike.
Stretch early, stretch often.
Cheers,
Jamie
P.S. I do mostly leg and back stretches of the usual kind--calves, hamstrings, quads, groin, etc.