Yes, I was/am concerned about DIM losing its effectiveness, but I found that if I up the dose by 50mg when I hit a plateau then it keeps working...and then I lower it back down to 100mg. I tried 50mg for a while but I think 100mg is the right dose for me right now.
Also, I had lowered my inositol to about 200mg-400 per day, and noticed that the melasma was getting darker...now I upped it to the regular dose (about 800mg) and the melasma seems to have faded a bit. It's crazy how my body reacts to hormonal supplements & herbs. It seems that I need both inositol and DIM, which is fine since I'd rather take those than birth control or anything else.
I've also been minimizing grains lately, and if I had more discipline, I think it would help my skin a lot more. The optimal PCOS diet doesn't have many grains anyway (perhaps except rice and millet), so I'm trying to stick to it as much as possible. I still have some little bumps under the surface of my skin, but I noticed they got much better when I upped the inositol to 800mg...I've also been wearing less makeup and did a lactic peel recently, which could have helped too.