And presuming they would release 1 class each 3 months ... it would mean that first 4 classes get aproximately 24 months of testing ... while last one will get 3 ...
Not exactly best situation. :-/
But irellevant, presuming they will not include desired changes anyway (Expertise, Scrolls, Spells preparing, etc.). :-/

Personaly i believe that the best approach from their side would be to release all 3 remain classes now ... that would bring lot of life to this kinda sleeping comunity ... and prehaps some content update (new zone) ...
Next patch should be focuseed purely on features ... or maybe add some missing subclasses (and we still miss a lot, especialy for Wizard) ...
That should keep us entertained for aproximately 8 months +/- ...
Third (or Last for EA) patch would be a little harder nut to crack tho. :-/
But so far the most popular patch was 5 (if i remember it corectly) and that was pure mechanical one.