There is no real definate answer for you. I would say that if you do the rolling pin on your own time, say at home, then I would spend about 1/2 hour doing it. If you do it during class, I'd spend between 5-10 minutes doing it.
How long does it take for your shins to condition? I really don't know, but when you find out, let us know. lol
What I mean is that it depends on a variety of factors. Depending on how your work out, how often you work out, and depending on your own bodies ability to react and adapt it is hard to give you a definate answer.
I can't even tell you how long it took my shins to become conditioned because while you are training, it isn't really something you pay attention to. Oh sure, you pay attention to the actual conditioning, but testing it out is another matter.
I didn't realize until about 2 years into training that my shins were well conditioned. You just kind of find out. I was sparring with Master K's son-in-law. We were going light with no pads, but I still cracked my shin right on his knee during a kick. I didn't feel a thing! THAT'S when I realized that my shins were conditioned.