I’m flying SBN-ORD-LAX in a couple of months on a Sunday afternoon. When I booked the flights, I had a 70 minute layover at O’Hare. My flight from SBN to ORD is leaving later and my flight from ORD to LAX is leaving earlier. So I now have a 45 minute layover. I haven’t flown through ORD in years. Is 45 minutes reasonable?
Sure, it's "reasonable". If you're trying to catch the last late evening flight (out of 10) to LAX then maybe not advisable. What's in your favor is it doesn't take much time at all for all passengers to get off a 50-seat CRJ-200. What can also be in your favor is checking a bag in SBN so you won't have any delay waiting for a carryon to be put on the Jetway at ORD.