For 5k races, most people feel good about finishing somewhere in the 20 to 25 minute range, as of 2014. However, a good time for a 5K race of 3.1 miles depends on a number of factors, such as age and running experience.
Male winners of a 5k race may finish somewhere between 12 to 15 minutes, and female winners may finish somewhere between 16 to 19 minutes, though these time frames are by no means set in stone. Walkers may take more than an hour to finish the race. A good way to determine a good time for a particular course is to look at the results of the previous year's race and compare one's age and gender to last year's statistics.