That depends on the complexity of your AE project and whether you are using an external monitor. With a monitor, on average mine heat up to around 50 - 60 deg celsius. For a very complex composition, sometimes it would go over 70 deg celsius. I'd normally use a wireless keyboard and mouse because it's uncomfortable to use and touch the macbook for long hours.

Without a monitor, the temperature is kept at lower at between 40 - 60 deg celsius.