Did you go the demodex specialist derm from Brussels, that I mentioned some time ago?
How long? Nobody knows, because nobody fully knows the cause. It's just trial and error. The only one from which I know who isn't using it anymore, is I myself. And maybe @anymnoo, he got back on it some time ago, but not sure if he's off again. And maybe user Firstman from another forum (but he hasn't reported back anymore since 1 or 2 years.)
I'm not using Soolantra anymore since December 2015. I'm only using Povidone Iodine as a facewash, which I would like to recommened to others to give a try. (Although I not fully sure to what extent this is helping.)
I still get new lessions, but it's just a lot less severe and less often than what it used to be, so that I can live normally.
Have you tried this? For how long?