Also -- keep in mind that if you're trying to get there in the morning it's much slower on Mondays. The traffic reporters specifically refer to the "Monday morning airport delay" which usually forms by 6am.
After 6:30pm there shouldn't be too much congestion on the way back, except on Fridays and curiously, Saturdays (suburbanites heading into the city for a night on the town).
At best it's a 20-minute drive between downtown and ORD. If you're driving add time between the parking lot and the airport. Personally I always take the El.