ParticleSystem.MainModule.duration

Description

The duration of the particle system in seconds.

This property can only be set 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();
    }
}