ParticleSystem.MainModule.duration

Description

The duration of the Particle System in seconds.

You can only set this property when the Particle System is not playing.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    private ParticleSystem ps;

    void Start()
    {
        ps = GetComponent<ParticleSystem>();
        ps.Stop(); // Cannot set duration whilst Particle System is playing

        var main = ps.main;
        main.duration = 10.0f;

        ps.Play();
    }
}