I bought mine about 3 months ago and it is still more than half-full. It goes a very long way, as I'll detail my usage below for you to better gauge:
I used it a lot at first [multiple applications per day, daily then again before bed], because I was having very regular breakouts due to the summer/autumn heat.
Then I used it as a "preventative" very lightly. For instance, I'd apply some on my forehead and chin where I used to break out. I'd only use a little at night; I probably did that a week or so.
Now, I use it more sparingly. I dust some on lightly if I think I may feel a pimple coming. I'm not sure if this actually prevented any breakouts, but I definitely don't need it as often as I once did. It is still just as effective, though, when I do have an occasional pimple.

I don't use other acne specific products anymore. Benzoyl peroxide and salicylic acid formulas are too drying for me by the time they diminish the pimple; this is so much more gentle and speedy. In terms of other products, I do still cleanse and moisturize daily, of course; some are BE products, some aren't depending on what I feel my skin is needing. I only wear my BE make-up a few times a week, though.

Sorry this was so long. I wanted to try and answer your questions well. :)