User experience makes questions like this tricky: graphics, sound, score-tracking, tightening up controls. If I was working with a graphics library I was already familiar with, I doubt the core gameplay would take me more than a few hours. Writing a reasonable clone of the version of Tetris I played fifteen years ago on the Atari ST would take at least a couple of weeks, though, and that's if I had all the resources I needed on hand. An exact clone would take even longer.

Depends on how many bells and whistles you want. For just a basic clone, I can do it in an evening, using a third-party library to handle the windowing and whatnot. Of course if you wanted to implement the windowing and input handling by talking directly to the OS, it would be an utter nightmare; call it three months.

If you don't care about graphics, like a couple hours. But you could spend as much time as you want on graphics.

An evening? Depends on what you mean by "Tetris" and the target quality. I did a two player Tetris-over-network for a class once (the opponent gets an additional garbled line when you clear multiple lines). It's easy, which is part of the problem: with enough expertise, it can become boring, you don't learn as much new stuff, it becomes more like laying bricks, not designing intricate machines or learning the principles of their operation.

An estimate that I heard on multiple occasions, disbelieved, and then witnessed come true, is that it takes about 4 years of hands-on experience for an enthusiastic smart adult to conquer the learning curve and as a result lose enthusiasm for software development in the abstract (so that you'd need something special about the purpose of the activity, not just the activity itself). I don't know about the probability of this happening, but the timescale seems about right.

A related question: how much would a beginner have to study before being able to write Tetris? As I said, I graduated with a degree in computer engineering without being able to code Tetris, because I have no idea how to write anything except console programs that use cin and cout to do all the input/output work. "Draw something on the screen, and have it move when someone presses a key" would seem to be a fundamental task in computer programming, but apparently it's some kind of super-advanced topic. :P

<brag>I wrote tetris for IBM-370/VM once, during school practice. The main problem was making the keyboard and display work in real time, had to write significant parts in assembler. Took about three weeks of evenings.</brag>

I did this (in Java) when I had some spare time at work recently. It took a couple of days of work (with some slacking off), including learning a (pretty simple) new game/graphics framework.

Use Pygame. Evidence: People frequently produce playable games using Pygame in 24-hour hackathons.