Agonal breathing usually lasts a few minutes after someone goes into cardiac arrest. Some people stop gasping within thirty seconds, while others continue for several minutes. Once the gasps stop, the person has gone into full respiratory arrest. CPR should already be underway by that point.
The length of time agonal breathing continues is not a useful measure of how long someone has left. The brain begins to suffer damage from lack of oxygen within four to six minutes of cardiac arrest. Survival drops sharply with every minute that passes without chest compressions and a defibrillator. Do not wait for the gasping to stop. Start CPR as soon as you recognise it.