In general, the answer is as often as you are comfortable. One of the contributors to the stigma surrounding mental health problems is the reluctance to talk about mental health in a real way.
However, what people are usually getting at when they ask this question is “How will know when to talk to someone about their mental health if I am worried?
We still provide a similarly simple answer to this – whenever your gut feeling says so. One of the most common things that gets noticed in social, work, school, or other groups is when there is a change in someone’s behavior, especially if that change is them being more withdrawn from others or their normal activities.