How long do I need to use my pepper spray, for it to be effective?

If you need to use your Pepper Spray or Pepper Gel for personal defense, your interaction should be as short as possible. 

While aiming at the assailant's eyes, press the activation button 1-2 times, using quick 1-second sprays. This should be enough spray to affect the assailant. But, if they are still pursuing you, you can use additional short bursts of spray as needed.

As soon as the assailant is distracted, you should get away and find help. Once you've used your pepper product, it is important to replace it. This way you are prepared in case you need to use it again in the future. If you've registered for Guard Dog Security's Protected 4 Life program, you can submit a usage claim here. Once your claim is received by our team, we will send you a 100% FREE replacement of your pepper product. 

Source: https://www.scribd.com/document/283341082/NYPD-Guidelines-for-Use-of-Force-in-New-York-City

(NYPD Patrol guide, page 69)