Obviously, Flash does not take that much time to cast at all, on account of it being a speed ability. Once activated and targeted within the range on the cast, V will near-instantly teleport to the selected location, either forwards or backwards.
Although it does not deal damage and cannot be used with the V is possessing a unit, Flash has some very useful potential to protect the V from being overwhelemed. The V cost 1350 gold and 100 mana so teleporting the V out from a cluster of Speartons, or ninjas can save the V and then the V can simply move back into the fight with ease. Also, if there is a particular target that V wants to attack, but something tough is in the way like a Giant, Flash will get him right past it. The V can also Flash forwards to get into range for possesses from a much further distance than the Possess spell provides by itself.
Multiple Vs can completely bypass an army, go for the Statue, and when the army catches up, Flash away. As well as skirmishing with Vs using Flash to escape you can also the Flash to bypass Order walls, though this will leave your V very vulnerable afterwards but your V can still walk back away from the enemy statue and pass through the wall normally as it only blocks entry not exit. Protecting Vs teleporting over the walls with Protect from Cycloids may be necessary to stop the V from dying to focused fire as you will not be able to get any other units past the wall to tank for the V.