And to answer your last question--I'd think assuming fast learner, and you're already competent with python, I'd think a few days and you can have up a basic app (blog, wiki, something like that), and a few weeks and you'll be pretty comfortable with any normal CRUD app.
Comments
I think if you end up in development as a career, Django is going to be used incredibly more often than Flask.
In the end, you will learn the internals either way. If you start with Flask, you'll still have to learn about Django internals.
Start with Django (and you'll probably never switch to Flask).
And to answer your last question--I'd think assuming fast learner, and you're already competent with python, I'd think a few days and you can have up a basic app (blog, wiki, something like that), and a few weeks and you'll be pretty comfortable with any normal CRUD app.