Can we try finding a consensus on this? What game comes to mind as having the shortest single player campaign?

If I remember correctly, Super Dodge Ball took me 60 minutes to beat.

Plenty of old-school games can be beaten in a matter of minutes. Contra for example takes just 15-20 mins start to finish for a skilled player. Mario 3 can be beaten in roughly the same amount of time using the warp whistles.

I remember my brother beating the original Super Mario Bros in like 7 minutes or something.

As far as current retail ($60) games? I would say take your pick of any FPS out there no?

any COD campaign

That'd be hard. Mostly because how can you measure something like that? Someone might take 10 hours to beat a COD campaign whereas people can beat Uncharted in less than six hours.

There are plenty games from the 8 bit and 16 bit era that can be beaten in less then 30 minutes even if you are not really trying to go for a speed run.

CoD: Black Ops Declassified was blasted in reviews for a campaign that was under an hour.

I mean, Far Cry 4 lasts 15 minutes.

The problem with this thread is that the answer will always be some old game. As you didn't had ways to save (except expensive batteries on cartridges), games were designed to be short so you could finish them in a single session. Of course, they were hard and cryptic to avoid being finished in the same day you bought. Modern games will normally be blasted if they feature 6 hours campaigns (without multiplayer).

I remember the first game I played on my first console, Sonic for the Master System. In the beginning, it took some time, but the last time I turned the Master System on I just beat it in 40 minutes easily.