At the latest, during the Steam Summer Sale!
It should start later this month...
If it does go on sale during that time, though, do NOT buy it unless it is one of the Day Specials though!
During the sale, a big bunch of games are on sale, but each day they select a few games that have an even greater sale. So, for Civilization, they might have 50% off usually, and then, during that special day, have 75% off... Brave New World won't have such a big sale, though - it was just released...
If the Summer Sale is reaching it's final day, and Civilization was never a special, then feel free to buy it (on the last day only!)...
Hope it helped!