As a client, let me offer a more precise definition: as a general rule, I think a BFE is at least in the offing when 50% or more of the session involves the escort doing something other than trying directly to get me off, and that time is both pre and post nooky. That could consist of: talking, hugging, caressing, kissing, watching TV with his head on my chest, napping together etc.
What is it not? When the guy walks in, drops his pants and bends over, or starts to jerk you off while you are still trying to get your shoes off. I've had a few of those.