How long is a piece of string ?
Edit: without kidding. It depends on smart you are and how fast you learn. It depends of whether you have done programming before. It depends on how good of a programmer you already are.
It depends on what you exactly expect from someone who "knows" python. Do you mean someone who knows the syntax ? Someone who knows all exceptions, weird cases, pitfalls and such ? Do you mean someone who can build their own AVL-tree, priority-queue or even build his own relational database ? Or do you mean someone who can program "hello world" ?
Going even further: what do you mean by "days" ? Someone who spends 2-3 hours per evening, but not in the weekend ? Or someone who spends 10 hours a day, even in the weekends ?