The runtime for a tank of fuel can vary depending on the conditions that the car is being used in, but it will typically be between 10 and 15 minutes per tank on most nitro cars. Don't forget that you can refuel with the engine still running so you can enjoy the car for longer periods without stopping!
1/8th scale models will generally have larger capacity fuel tanks on them,but because they will normally have larger size engines you will get around the same runtime as 1/10th scale models.