According to this study, the minimum need for nature is about 2 hours per week for people to maintain good physical and mental health. That can happen all at once or be subdivided, such as two 1-hour sessions, four half-hour sessions, or a little more than a quarter-hour each day. This is the first time I've seen a concrete timespan attached to health markers, as well as acknowledging that adults as well as children can suffer from nature deficit disorder. But it's pretty obvious that if people get little or no access to nature, they tend to get sick.
In pursuit of this, a daily walk of more than 15 minutes can fulfill your nature needs. For most people, that's about one mile. This works well for folks whose days are equally busy.
Longer (two 1-hour or one 2-hour) sessions of forest bathing may work better for people who have less wiggle room in their daily schedule but more time on certain days.
Here are some other outdoor activities for kids and adults.