For First Timers: Double your half marathon time and add 20 minutes. This does require that you have done the proper training, long runs, etc. If you are unsure, it is always best to err on the side of caution. You can always pick it up if you start to slow, but if you go to fast from the start you will fade earlier and end up losing much more time later.
For Seasoned Runners: Double your half and add 10 to 15 minutes, based on how your training went.