I'll break your question in parts.
Question1: How do I learn to represent the features with a few lines instead? [like in comic book or animation style]
Answer1: Find comic book or animation artists that draw the way you want to draw. Try to get pictures of their face drawings from all angles you can find (Pinterest and 'comicartfans.com' are good places for that. See how they simplify the face and its features, and try to redo their drawings. First you draw them from the same angle as the original drawing. After you get comfortable with that, try to draw the same face as the original but in a different angle.
Q2: how do I actually make the transition? How long do I need to practice slow studies before going into simple heads?
A2: I don't think this transition have to be linear. Try the two approaches, detailed and simplified, in parallel, until the style you want to draw becomes easy to you. But if you get in trouble, in the simplified, with some angle or feature, go back to the slow studies until you solve the problem. The two approaches have a lot in common, and what you learn in one will never be useless to the other.
One more thing I would like to say is that, no matter how simplified your approach of the head will be, every head drawing should always have two things: solidity (it should look 3d) and simmetry in features placement (you don't have to follow no idealized proportions of the features placement, but it must be simmetrical)
Hope this helps.