I am a US Citizen and can get a 10 year visa.

1. The length of each stay depends on which type you are applying for, it can be 60 days, 90 days, 120 days, or even 180 days.
2. It's possible that you can extend it in China seven days before your allowed stay running out. The extension period can't be longer than the initial allowed stay duration.
3. As your wife is a Chinese, your chance for getting the 10-year entry may be bigger.