It’s hard to determine exactly how much yoga is enough to obtain health benefits. But one of Greenberg’s studies found that people prescribed the highest amount of yoga home practice, four hours per week, achieved significant stress reduction, even if their level of compliance was lower than the groups with lighter prescriptions of two-and-a-half hours and one hour weekly. But it is possible to go overboard and end up discouraged, Greenberg says. Forty minutes a day is a…