This is a difficult question to answer because it varies from person to person. Iv gone through this process and truly know how difficult it can be to have the patience to wait for the medicine to work. I say that your purging stage should be over within a month. What bothered me about this waiting stage was all the red marks that would accumulate on my face since once the pimples go away they often leave red marks that stay on for a while.

I wish you good luck

For me, the purging stage took about three weeks for everything to heal. I don't know if this is the case for you or not, but I got really bad cysts while I was purging and I found this great cyst regimen online. So first you ice it for 10 min, and then take it off for another 10 min and just keep switching between on and off for an hour. Then apply benzoyl peroxide (2.5%; avoid 10% because it will dry the skin out too much) and wait for it to dry and then apply hydrocortisone cream (I used an anti-itch cream which had 1% hydrocortisone) which reduces the size and inflammation. You will probably start to see a head the next day! Do it once a day and the pimple might be gone within 3 days  

Also! A great acne product that works wonders on cysts and just regular pimples is Murad Clarifying Mask with 4% sulfur, which is great for acne. I use it as a spot treatment and just leave it on overnight. It is pricey at about $40 but by far one of the best acne products I have ever used!! You can get it at Ulta or any beauty supply store... I found mine at Meijer surprisingly! 

Make sure to change your pillow case every night, especially if you sleep on your cheek. I have 4 pillow cases that I rotate through during the week, and instead of having one pillow case for each day, I just flip over the side I wasn't using the night before. After I have gone through all of them, I just wash them like I would any other laundry and use them for the next week! 

Also, something I did that was really helpful mentally was doing video diaries every week while on differin and anti-biotics to see my results. It really helps to see the progress and how your skin is improving over time. My acne was fully gone in about 5 weeks and I only get random flare-ups now from stress or hormones. So hang in there!!! You're going through the roughest part right now, but it will all be worth it when you have clear skin in a few weeks! Good luck!!

Thanks for all your advice and comments guys! So helpful. I guess I just have to focus on hopefully the results at the end of this rather then how it looks now. They spots pop up the day after applying differen and they are sometimes really painful ones. I've been using Duac during the day or neturogena 2.5% benzoyl peroxide to help, but I will definitely try the ice. I try not to wear makeup when I don't need to but my acne is a really bad insecurity of mine ( as I'm sure it is with most people ) - like a lose lose situation