When you're playing the Arbiter missions, proper cloaking is key. The time you remain cloaked changes depending on difficulty.
Easy/Normal: 8 sec
Heroic: 6 sec
Legendary: 4 sec
Now that you know the time, use it wisely. When you're cloaking, remember that your enemy can see you before you cloak, but he may not fire upon you. If he sees you, acknowledges you (pointing, jumping in shock, etc.), you can cloak and get away before they shoot you, and be fine. Also note that you cannot cloak in the heat of battle, or when your shield is down at all.