So, wonder what this mysterious "northern army" is, and when it's going to arrive? Wonder if Xykon's going to splut Redcloak for developing a conscience (of sorts)? Wonder if we'll see what's under the unbrella anytime in the next 100 strips (or if it's revealed in the new book, which I honestly doubt)?
There's no mystery, it's the army at the northern end of the city. The one attacking the breach already where Vaarsuvius is. Look at the first panel of #450: the bottom army is the "southern army" scaling the walls, the top army is the "northern army" storming the breach.
At least three minutes, assuming he isnt using a metamagic rod. Its an 8th-level spell, so he needs to be at least L15, so that would be 30 rounds (3 minutes).