// 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 Awake() {
particleEmitter.emit = true;
yield return new WaitForSeconds(3);
particleEmitter.emit = false;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

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