I never take D-bol longer than 6 weeks as I've heard it is/can be hard on your liver. I had my levels tested and they where all still in acceptable ranges after 4 weeks, didn't check after 6. But IMO, why chance it?
If your takeing Dbol alone (I wouldn't recommend that) I would suggest taking at least 30mg/day, but even that's kinda low when your taking it alone (only 210mg a week!). There is a lot of debate about once a day doseing vs every 4 hours (Dbol's half-life is 4 hrs+/-). I think spliting it up into 10mg doses would keep your T levels more constant through the day. But I have heard that a once a day, AM dose is less harsh on your natural test production. I always used an injectable with it and since I train in the early morning, I opted for the once a day thing. It really gave my workouts some added intensity.
I would ALWAYS have an anti-estrogen on hand or an aromatase(Sp?) inhibitor like Aramidex when useing a steroid that aromatizes. I always take A-dex when I'm useing dbol. I wouldn't even consider starting a cycle with out one. Also do you have any sort of PCT planned?