Shortest feasible commute?

2 miles is the shortest for me (and my current) despite the six roundabouts and three junctions (incl minor to major which can be a bane turning right) I can do it in abt 8mins normally and under 7 has been done if I get a move on and don't have to slow too much for the various motorist carp that crops up.

I used to cycle to my step daughters regularly that was 400metres away and the post offic about 600m, I keep a bike in the hallway perm and slipping the PDs on is actually quicker than putting normal shoes or trainers on and honestly I'm more often to be found wearing cycling apparel anyway so walking isn't faster even over 400 yards for me, not even close.

Generally, anything that takes more than 20 minutes to walk is worth cycling, providing as others have said there's no major faff in getting ready to ride, or parking the bike at the other end.

My commute is just over a mile. I'll walk if it is icy and have the option of a nice 5 mile offroad to come home if it is sunny. Like you, I often go home for lunch. It takes about 5 minutes and I cycle in work clothes.

I know of people who work in the same office who think nothing of jumping in the car every day to drive less than 0.4 miles. I'm pretty sure they could walk home in the time it sometimes takes them just to get out of the carpark.

The simple rule surely is whether it takes longer to get the bike out the shed, ride it, and lock it up at the other end.  

For me the 15 minute / 1km walk to the station is quicker by bike.  On a hot day there is an added sweat avoidance factor.  I've never ridden whilst holding an umbrella so on a wet day I'd walk to stay drier.

Isn't there a long way round?

but you do have a power meter yeah?