If I was the amateur guy facing MLB pitching the question in the OP wouldn’t ever be interesting.
A better question would be: “How long until I even make contact with the ball?”
Looks like someone wrote an article inspired by the same tweet that inspired me to pose this question.
The gist of the article, if you don’t want to read, is the Average Joe would probably get one or two hits in a season.
When I was in college I was a demon slow-pitch softball player, but had never played organized hardball. But when I saw a flyer posted that said that our college team (and we’re talking major, Division I big-time athletics school) was going to have open tryouts, I thought “How hard could it be?”
I got into a batting cage against a pitching machine. not a MLB pitcher, not even a college pitcher – a pitching machine. I whiffed 10 straight times and slunk away in shame.
So to answer the OP: I think it would take forever.
A live pitcher has a lot more cues for generally timing when the ball is coming than a machine, unless it’s one that gives a cue.
The object of a pitching machine is to let the batter hit. It’s no secret when the ball is coming.
First the pitcher - If he has made it to the major leagues he has a variety of pitches. If he want to get that batter out and is not just having fun it will be next to impossible to hit him. Fastball, fastball, curve-ball, you’re out. Miraculously foul one off and a sinker will sink the batter.
Getting around on the pitch - Go to a batting cage, set it at 60mph. See if you can get the bat around on the ball. Difficult. Set it to 70mph. Far more difficult. Set it to 80 which is the speed that a slow curve ball will come in at. Can you even see it? Up to 90 mph, the low end of a major league fastball. Almost impossible. If you are lucky enough to get the bat around and make contact it will only be fouled off. 100mph, you are at the lavatory trying to clean up your underwear.
If a major league pitcher wants to get you out, he will. When the batter can get good enough to deal with the challenge, I don’t know but it will take years of practice.