It obviously depends on several factors, but there is an average speed.

For an adult of average physical shape, 10-12 minutes is average.

Athletes with training can run a mile in 5.5 minutes or less, depending on what they are training for.

A 'four-minute mile' used to be thought impossible, but is now something that seriously trained competitors do with regularity.

As of now, the world record speed is 3 min 43.13 sec.  It was run by Hicham El Guerrouj, in 1999 in Rome.

So to really answer "How long does it take to run a mile?" depends on whether you are a child, an Olympic athlete, an average adult, or a senior citizen.  But the average is still between 10 - 12 minutes.