Majoru Oakheart said:I can tell you that on average encounters seem to take about 1 hour in 3.5. Whenever we write adventures for Living Greyhawk we tend to use 3 encounters as a baseline for how much combat to have during an adventure mostly due to time reasons.
They are scheduled for 4 hour time slots and 3 encounters tends to take around 2.5-3 of those 4 hours.
So it doesn't seem like a huge change in the time to finish an encounter.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.