This has been in the game since launch and has not been addressed properly. I know that Bioware claims that it takes a consistent time of 10 seconds or so to disengage from combat.
Anyone who PvPs and has half a brain would understand that this statement is completely wrong and only works in theory. It can take anywhere from 5 seconds to 30 + seconds in order to get out of combat. I know that placing DoTs on someone and running away will not get you out of combat, and having DoTs on yourself will not get you out (most of the time).
But honestly, after killing someone and not having DoTs on myself or anyone else, it is ridiculous how inconsistent the time required to disengage combat really is.
I have never seen anything so inconsistent since the rez door (don't even get me started on that bs/bugged RNG mechanic)