I wouldn't recommend anything longer than 8 weeks. 12 weeks may be too long for me(from experience)
People are so scared about losing muscle...12.5mcg is ANABOLIC not catabolic..on this dose my muscles were rock hard and my lift actually go up at the gym, great muscle pumps...5-6 trips to the bathroom a day.
Higher doses do not increase protein synthesis from the research I've read (100mcg). This even suprised the researchers. Just use 12.5mcg and you will feel and see what I'm talking about.