After searching a little bit (well an afternoon in fact lol but not all on this spell
) I saw that the spell last only 10sec with 45s cooldown, big help sure but very temporary.
I never thought putting point in staff mastery as I mostly use hit and run/freeze with fireball / lightning spells (as I suffer a lot at contact ... surely like all warlock ... ). I heard that staff mastery effect are cumulative ... I'm going to look this a little.
Edit : Question again !!! (sorry :O). Reading the description, Time Master seems to be an interesting spell, doesn't it ?? A reason explaining nobody take it ?? Is it only because warlock are damage maker and people prefer Arcania and Elements more than Mental or does this spell is not as good as I think ???