Where are you in story progression?Take it slow, there's no hurry.
Act 5 isn't going anywhere pace yourself but to answer your question the answer is zero units if you save up all your 4hr crystals...... my second account had over 900 crystals they don't spoil or disappear only use 150 or so to beat the collector I have over 3000 crystal in my stash same for main account going for caviler...use crystals to get health and revives!! now with the second account I'm going to complete act 5! Good luck GRIND ON!