When is the right time to quit, if at all (2-year Master's)?
I think that it really depends on how much you're struggling. Are you failing your problem sets? Are you merely unhappy with the courses?

I expect that there will be huge variations with the answers, but personally I would advise you to finish out your MS. The reason being that there actually is a huge difference in terms of your career (salary, promotions etc) when you have an advanced degree, so usually an MS is a worthy investment (as an aside, there are studies that suggest that having a PhD does not offer a lot of extra advantages compared to an MS, so if you are already unsure of your MS, I could advise against a PhD).

Another reason is that it would not look too good to your employers to show that you dropped out after just a semester in your program. Perseverance is a very strong quality that is appreciated by many people.

Furthermore, your experience depends largely on your professors. I would talk to other students to see if there are particularly popular professors who teach courses that you are interested in, and try to take those classes next semester (and while you are talking to your fellow grad students, you might as well ask them about your current feelings and ask if this is common).

That being said, I suggest that you talk to your program director, because maybe your program is notoriously hard, and there are many students feeling this way. You may walk out of that meeting feeling reassured, or maybe that meeting will give you a very strong and concrete reason to drop out of your MS. Or maybe the program director could suggest an alternative MS program that can still credit this semester towards the degree (for example, it sounds like an MS in computer science might have been a better fit than an MS in statistics, but I can easily imagine an MS in computer science requiring some statistics courses).