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Use it as often as you can, 50% is fine but if you think your gonna be getting to a boss soon and you may need it try to save it for then. Too many resto shaman's get this and only use it when they are oom on a boss fight the cooldown is not that long and it can be used to speed up pulls.
If your soloing, drop it when your getting low on mana, like around 20%. It will help you to not drink as much.
Generally I will drop my trinket from ST, then my mana tide (longer cooldown), but now that im at level 64 and have water shield, I rarely use my mana tide. My water shield will keep me going forever IF i can manage to remember to keep applying after each fight.
So in other words, use it when your at 20% mana, and your soloing. When you main healing a group, drop it when your in tight spots. :) I hope that helps.