You did ask about a half marathon, correct? Please notice that most answers are giving estimates for actual marathons, not a half marathon. A half marathon is 13.1 miles, and the time it would take depends mostly on the condition, experience, and routine of the runner. If the person knows approximately how long it takes him/her to run a mile, then multiply times 13, and that will give you an idea of approximately how long it may take to run it. Good luck and enjoy!
Usually a mile takes 8 minutes for an average runner. If you multiply that by 13 you get about 104 minutes or 1 hour and 44 min. So if you are a decent runner it should take you probably about an hour and forty five minutes to complete.
A marathon is 26.2 miles. Anything else is not a marathon. Other runs are almost always shorter runs. I ran distance here and there over my younger years. I ran a 9:58 on a two mile run, so if I extrapolate that it would be about 2 hours and 10 minutes for completion of a 26.2 mile run, if I could have maintained the pace over this much longer distance. At that time in my life, I could have maintained the pace for about 2 more miles, probably less. I never ran more than about 5 or 6 miles for speed (time). I was too short to run longer distances for time, but I could definitely run until my shoes gave out. heheheheehe