Fighters and bombers thrust faster because they are lighter, and thus weaker than cap ships. That ofcourse doesn't give them liberty to just thrust away from the fight all the way out of radar range and consider themselves still in fight.
That DOES mean though that they CAN thrust away to 6k, recharge their shields while for example, their wingmate keeps the damn cap ship busy. Thats called teamwork and it works great.
So.. To answer question above, if fighter starts thrusting away and stays within some sane range (8k, 6k) I don't consider him fleeing. If capship enters cruise then, the capship lost the fight. end of discussion.