For me, yes, the rule is the same. RQG p390: "For each point of POW added to the inscription, the base strength, range, or duration of the spell is increased by 1, without requiring any manipulation by the sorcerer."
Nothing says that is must be in one shot. In fact, the only difference I have noticed is that by default, the RQ3 sorcery spell matrix could be used by anybody, as the RQG inscribed spell can only be used by it's creator.
I was doing it, just because it was the only way to obtain the long duration needed to have all your spells up when you need them.
Whatever the case, RQ3 sorcery were longer to cast than other spells just because of the sheer number of MP needed. It is just longer now. Note that it does not bother me, because I was already using my sorcerous characters that way, and when you cast a 1 season duration protection spell, you don't care if casting it takes 4 or 5 rounds.
The casting duration is only important for direct, personal attack spells. For those, I use spirit magic spells (in matrixes), a bow and a broadsword, wether RQ3 or RQG.