An 8k race is 4.97 miles. The length of time to complete the race is dependent on the runner's pace. It is possible to make an estimate based on how long it takes a runner to run a mile and using a race-time predictor calculator.
A race-time predictor uses a runner's age, gender and time of a mile run to help determine a race's outcome. Even with appropriate training, there is no guarantee of a runner hitting a certain predicted time. Other factors, such as experience, overall health, weather and course elevation, can cause a change in pace, resulting in a different completion time than one previously estimated.