I am assuming that you are talking about leaving it connected to the engine via the glo-plug. If this is the case, it is usually necessary to leave the ignitor plugged in only long enough to start the engine. Sometimes leaving it on a little past the engine start helps.
Sometimes, you may need to leave the glo-ignitor on a little bit prior to start in order to get the engine warmed up, but it is much easier to use something like a hair dryer to truly warm the engine up. This will save the glo-plug from being burnt out.
Other than that, you will find numerous stories of people who have actually ran their RCs with the glo ignitor on for quite a bit...of course they do a big awww sh... when they realize it. The ignitor is not the most aerodynamic of items when sticking out of the top of your RC.
If you are looking for an exact number of minutes or seconds, you may not find any solid answer. Why? The length of time required may very with each engine and the ambient conditions (outside air temp, humidity, etc.)