90 minutes is the optimal duration for achieving certain types of immersion: social, narrative, health, entertainment, and more. Any shorter than 90 minutes, you cannot cover all the bases. Any longer, the brain may lose focus.

Generally, I set aside 90 minutes for coffee or meal get-togethers and tend to hit that mark without keeping track of time. All of the bases have been covered and the situation has turned cognitively stale. 

While being a conceptual and social theory, the 90-minute rule may be naturally linked to the circadian rhythm of our bodies (a sleep cycle lasts roughly 90 minutes, for example).

I have found the following to be most effective when conformed to a 90-minute window of time:

• Revisiting with an old friend
• Business meetings
• Brainstorming sessions
• Feature films
• Home dining experiences
• Concerts
• Board games

With enough arcs and nuances to an activity, the 90-minute rule can be broken and expanded to achieve longer sustained immersion. Hikes, conventions, recreational sports, and several forms of entertainment can all present enough twists and turns to keep you invested longer than 90 minutes. The average duration of my ten favorite films, for example, is 131 minutes. Rich and fulfilling content or activities can transcend time (and your day calendar).

Can you think of any other activities that fit a 90-minute profile?