I searched and I couldnt find this, So anyway, If you come out of a battleground you know you have PVP flag on for another 5 mins.
What you can do to get rid of this flag much earlier is to get on a gryphon flight. It automatically takes the flag off.
Uses for this are if you are going to an instance and you dont want to be flagged for fear of not being healed. Just get on a bird, and then ask for a summon.