There is no fixed rate of exchange between activity points and BRO. It depends by number of people listening to radio stations in the given interval of time.
I will try to explain more detailed. For every second you listen to radio you get one activity point. Every hour you will get a payment = your number of points/total number of activity points.
That amount of BRO is moved to your temp balance; when it reach 0.1 BRO - sent to your wallet.