I just tested with a "push button like crazy" rotation, keeping most abilities with mana costs on cd (e.g. taunt, DivProt, AS...
but not Cleanse, HoF, HoP, Salv, Sac, Hoj, Blinding Light). In addition I did not use any fillers without mana cost (SS, lvl90 talent, AD, GoAK) to get the most mana per GCD output. I managed
finally to go oom after 6:30 minutes! I particular went to Darnassus in order to get a low latency (80 ms in my case). Under those circumstances I still doubt anyone will have to deal with this problem under normal conditions. You can manage to do so but you really have to work toward 0 mana, e.g. using Cleanse on cd.
Do not compare this to the situation in 3.x, when we almost only relied on
Spiritual Attunement. In those days a prot pally, who didn't get healed by someone else went oom in seconds. I'm still more concerned with blizzard's ambivalent attitude towards stat distribution on our tier set for prot.
Exactly that seems to be the case for some devs.
PS:Thanks Valarius for this excellent addition to this discussion.