It can stack up to 4 and even proc again at 4 stacks but will only refresh that stack when it happens. Cannot stack to 5.
Getting it to stack is pure luck. I commented on this a while back:
It's still a great trinket but there's no reliable way, other than chain-casting, to get it to stack. Often you'll get another proc just a second after the first fell off. Simply bad luck, I'm afraid.
Originally Posted by Venara
Here's a few numbers from last night and last week. I'm healing mostly with atonement.
Boss Mana Fight Mana Fight
Jin'rok 20625 03:18 56375 03:35
Horridon 78375 08:55 96250 09:19
Council 108625 06:26 75625 06:39
Tortos 42267 04:42 59841 05:42
Megaera 94875 07:23 63250 08:12
Ji-Kun 110000 07:02 123100 08:58
Durumu 95342 07:45 112750 08:52
Primordius 71440 07:24 74250 07:57
*Edit* I have the LFR version at the moment. Normal won't drop :S So these are numbers from the LFR version.