Well, depends. You want a burn-out to show off, to heat tires, or clean tires? Is the car an auto, or a stick? Many factors can contribute to the cause.
To hear up tires, a good 7-10 second burn-out will suffice. To clean some radial tires, a 2-3 second burn out is more than enough. To show off? Well, you can hold until your tires catch on fire.......literally.
On an auto: Hold brake, and give it gas until the tires break lose and hold it until desired.
In a stick: Rev up the engine, pop the clutch, and quickly hit the brake with the left foot while pressing the gas with the right to stop the car but not hard enough to stop the tires from spinning. Hold until desired.
If you have a Line-Lock system, that makes burn-outs much easier and less stressful on your cars' drivetrain.