About 20% a year some say. If you are going to stock up on batteries for an emergency or set up a BOB etc it is probably best to check it every year I think at the least depending on what you have contained in it. If I'm going to store batteries that don't have an expiration date I note the last time I charged them. 20% of 3000mah would leave a battery at about 2400mah and a 3400 would be at about 2700mah or so. If you were to check them every 6 months and top them off the discharge would be about half as much.
The easiest thing to do is to just buy 20% more batteries if that much discharge rate is a deal breaker and also make sure to include chargers for every type of power source like home and auto (12v) and if possible USB also.