You can expect a journey which is slow and wonderful, difficult and fun, frustrating and fulfilling.
I have been teaching myself Greek for about five years, averaging about an hour per day, every day.
It took me about two years to get to the point of being able to read the Greek New Testament. After that, there is a point of diminishing return for that particular goal. I've done much since then, reading widely outside the Greek NT, and focusing on listening to, writing and speaking Greek, but I think my ability to read the Greek NT as I would English has not improved as much as you would think. What I mean is, to reach that particular goal is not that hard, all it really takes is working through a few text books and then reading and rereading the Greek NT over and over again. The Greek New Testament is really much easier than most Greek. Plus, you already know what it means in English. I also think that reading outside the Greek New Testament does not give you much insight into the Greek NT. The Greek NT is sui generis. If you really want to master it, it is a large enough body of texts that just focusing on it will do the trick. The true value for reading outside the Greek NT is that there is some stuff worth reading on its own, above all Homer and Plato.
Now, to master Greek in general, to be able to pick up a random text from Plutarch or Aristophanies and read it as well as you can the Greek NT, well that is a massive enterprise. FIve years later I have made a lot of progress, but not as much as one would with five years of an hour of Spanish every day. Ancient Greek is extremely hard. It's fun, there is nothing like it, but it is hard.
Once you finish Mounce, I would say that you are 30% toward your goal of mastering the Greek NT and 5% toward your goal of mastering Greek. You will not be able to read much outside the Greek NT. Maybe a little LXX and some of the easier Apostolic Fathers. Even to read the Greek NT, you will probably have to work your way through a few more Greek text books. I would do Machen and Summers. Avoid intense grammatical analysis, Skip stuff like Wallace and Porter. Check out Zerwick's two books.
You asked about milestones. Your first goal should be to read through the Greek NT cover to cover. Your next goal should be to do this about ten times. The first time I read through the Greek NT, after about two years of self study, it took me eight months. Recently I did it in less than a month. During that time I continued to read other stuff in Greek. Your goal should be to be able to read the Greek NT cover to cover in two weeks. You should also be able to listen to the Greek NT read aloud and understand 80% of it.
If you want to know what to read outside the Greek, go to this sitehttp://www.vnoel.com/component/option,c ... emid,1068/
Stick with it. Stay motivated. There is an old saying, it's hard to be a Jew, but it is good to be a Jew. Greek is hard, and it is good.
I am writing in Ancient Greek not because I know Greek well, but because I hope that it will improve my fluency in reading. I got the idea for this from Adrianus over on the Latin forum here at Textkit.