Not terribly long. I had the basics down in an hour or so, but I don't shuffle very much because it tends to aggravate the tendons in my hands. I played video games a lot as a kid to the point that I think I may have done some physical damage. This was back in the day nobody had ever heard of carpal tunnel except for in slaughterhouse workers. So I grabbed another Coke and kept playing.
I did mention it during one of my high school sports physicals and the doctor said that while I don't officially have CT, based on my past lifestyle and the symptoms I was describing I'm definitely in the higher risk category to develop it. He gave me a few exercises to strengthen the muscles around those tendons and left me with the basic advice of if it starts to hurt, stop doing whatever it is. It's been 20 years and that advice has served me well. Arching my hand, particularly with more than six chips in the stack, will make it start to hurt after a few dozen lifts. During an evening of play I might do five or six shuffles and that's plenty.