I've flown out of Brussels a few times since last summer, but usually on the Chicago flight at about the same time. Check in was OK but there have been pretty long security lines if you don't have access to the fast track line (which I figured out I was eligible for the last two time, but wasn't aware of the first of the 3 trips). So if you are not eligible (business class?), than I would make sure that I left a solid 30 (maybe even 45) min to get through security.