I don't hand them out per session. I usually give , as Mixster suggests, about 3 when the PC's have completed some sort of goal/quest and have the time to contemplate and learn from their experiences. If they were, say in a Dungeon environment, I would probably give out an IR for every 2-3 encounters depending on the challenge, whether they be roleplaying, puzzle solving or straight combat. It's your game though, do what seems fair to you and what you and the players are happy with.