If you select a collection and right-click > ETA on selected, the ETA info at the bottom will turn blue and show you temporarily ... well, you can guess.
The collection needs to be downloading, though. As I said above, what would be the point of ETA info for files that haven't started downloading yet?
EDIT: actually, I think you understand what you mean now: a column in the queue that shows, for each collection, the approximate result of (collection size / current speed) + ETA of previous collection, as a cumulative sum. I suppose you have to think about how ugly that could get with a long queue ...
