According to this Michigan study based on salivary biomarkers, the greatest stress reduction benefit is achieved with 20 to 30 minutes of exposure to urban nature. Participants taking a "nature pill" of 20 to 30 minutes saw stress reduced by 21% according to salivary cortisol levels and 28% according to salivary amylase levels.
Hunter, MaryCarol R., Brenda W. Gillespie, and Sophie Yu-Pu Chen. (April 2019). Urban Nature Experiences Reduce Stress in the Context of Daily Life Based on Salivary Biomarkers. Frontiers in Psychology 10: 722. https://doi.org/10.3389/fpsyg.2019.00722