Hi i've been on doxy on and off for a while. I went off on my own accord and not according to doc's advice. Bad move, i know. Anyway, it took about 2 weeks to a month for the doxy to work for me. I had almost no new outbreaks after 2 weeks, and the remaining ones began to clear up. About 1 month later, I had clear skin with only very occasional outbreaks. But I must say that when i stopped taking it, the acne came back within about 3 months.
Also, my doc says i respond well to doxy, so how fast this works for u might differ from me. I didn't respond well to mino, and that's why my doc switched me over to doxy.