I had a peek at the Mars Design Reference Architecture 5.0 [1], and under "trajectory options" they talk about two classes of missions. "Opposition class missi

I had a peek at the Mars Design Reference Architecture 5.0 [1], and under "trajectory options" they talk about two classes of missions. "Opposition class missions" stay on the surface for 30 to 90 days. "Conjunction class missions" stay for 500 days or more. That confirms the 30 days that you mentioned as a reasonable minimum. [1] https://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf https://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf