MST (minimum service time) for a 738 is roughly 45 minutes. Once the inbound actually blocks, that's when they'll update the new departure time, giving the ground crews that MST to get everything done and turn it. Generally speaking, they usually make it happen, unless it's unusually heavy. The biggest issue that usually delays things is getting pax on and off. Bags and cargo are easy.

It's hard to update the departure time beforehand because a lot can change. Plane swaps or the original plane coming in early. I saw one yesterday where the inbound aircraft left ORD 38 minutes late, and was only scheduled for a 55 min turn originally. It made up about 25 min in the air, and then got turned around at the destination in about 35-40 min, and ended up leaving early. So much can happen, you just never know.