1 every 10-15 minutes. A good way I've found to use them is in Crucible. I have 3 characters and pop a coin run a match on character 1, switch to character 2 pop a coin and run a match on character 2. Rinse and repeat for character 3. By the time I cycle back to character 1, those 10-15 minutes are past and I can pop another coin for character 1. I found that I get exotics every 3-6 coins, and it's pretty efficient if you equip each character with a favorite load out so you don't have to worry about switching gear. My only problem (besides being average in PVP) is that I will sometimes forget which character I'm running. One time was charging in to fist of havoc a group of enemies and shadowshot the ground instead...