// Emit particles for 3 seconds
particleEmitter.emit = true;
yield WaitForSeconds(3);
// Then stop
particleEmitter.emit = false;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    IEnumerator Example() {
        particleEmitter.emit = true;
        yield return new WaitForSeconds(3);
        particleEmitter.emit = false;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example() as IEnumerator:
        particleEmitter.emit = true
        yield WaitForSeconds(3)
        particleEmitter.emit = false