I’ve done well over an hour before (one way), but it definitely takes planning so not anywhere close to casual. At that point, perhaps other transportation may be used to return. And/or even if a daily/weekly occurrence, it takes discipline to do things like leave on time and stay informed throughout the day to handle weather events, plus wearing certain clothing to handle sweat and sun and rain etc. Biking is definitely preferable to walking for that level of distance/time, though I’ve done both.
More routinely, and perhaps with little notice, maybe… 20 minutes? (one way) It’s hard to say bc it depends on what stores are where, whether the purchases will fit comfortably into my bag, etc. But I’d leave at a moment’s notice for such a trip if need be and the conditions are conducive to it.
What about you: what’s your cutoff?
You could park in the furthest parking space as you walk into a store, for nostalgia’s sake:-).