The effects are really pretty bad after a year and significant even after 6 months with exercise. That's why it was so important to get the transit time down to 3 months.
Have there ever been any serious plans or proposals to do that?
You really want a pretty large diameter so that you don't have a huge gravity gradient causing your astronauts discomfort. Tethers might be more practical than the classical doughnut designs for the near future.