I think that 10 years is too long, especally if you plan on growing your hair out or getting rid of facial hair (maybe if you keep the facial hair you'll able to pass as a male when you want to).
If you want your spouse to stay, you should not hide this from her, that's just wrong, and if you know already that she won't stay, then that will not change in the future, and it's still wrong to keep it from her, because you're wasting her time. She has the right to get out of this relationship and try to find something else to make her more happy, if that's what she wants. Plus, if she was very liberal and wanted to stay together, she would feel hurt that you are taking hrt without consulting her first.