Let’s say that a place is perfectly familiar to you if you hardly notice you’re there. You can walk from one room of your house to another without using any mental resources to find your way, and if things haven’t changed, you hardly register the environment. I became so familiar with my Metro commute to College Park, MD–45 minutes each way for 15 years–that I often didn’t even notice changing trains at Fort Totten. I would have to look up to see if I had already shifted from the Green to the Red Line. Even accounting for some eccentricity on my part, I think this is a general phenomenon: We save our attention for what is new and requires thought.

I’m interested in when we reach a stage of familiarity. It doesn’t seem to take all that many hours. For instance, I would guess I spend an average of 2-3 hours per week on airplanes. Each plane is different. Yet that’s enough time to make the situation familiar once I settle into my seat. The context pretty much disappears and I can be fully absorbed in reading, thinking, or talking. So I think familiarity increases on a steep curve.

But then it matters how much time passes between experiences. My mother-in-law’s house in Georgia feels extremely familiar to me. I’ve spent no more than two percent of my time there since the mid-1990s, but our visits have been spaced fairly evenly over the years. Returning for a couple of days every 3-6 months seems to be sufficient to retain a sense of familiarity. In contrast, if we had spent three months there in 1996 and never returned, I’m pretty sure it would feel unfamiliar.

There is surely some individual variation in how we experience familiarity. I am not sure how I’d like to be in this respect. Attaining familiarity seems desirable, insofar as it reduces stress and distraction and lets us focus on our choice of tasks. At the same time, it seems bad when it makes us careless or inattentive to the world or when it makes time pass too quickly.