The results come so fast and furious that most people peak at 6 weeks and find that using it longer doesn't result in much change. I would suggest you aim for 6 weeks and then make your decision whether to continue or not when you get there.
I ussually run it 6 but my last cycle I decided to see how 8 weeks would be. Ulter called it right (again) the addtional 2 weeks did not make that much of a difference.