Good point about Cas's concept of time...I just assumed, given that Dean is the only ally he has in the world, that he would be in closer contact...Still, even with the 3 month gap, if we assume that the stories weren't happening simultaneously, then Cas could have visited Dean and done the whole archangel thing in July, and then disappeared for another month and arrived again at the end of August in time to save him from Zach...meanwhile, Sam's roaming the countryside getting regular jobs until Lucifer tracks him down near the end of August....

But I agree, the three months *feel* better...especially if we want to believe that the time apart gave them some perspective.