Maximum load current that is still indistinguishable from battery self-discharge
The only way to know the actual self-discharge rate of any given battery/cell, is by measuring from a fully charge state to a discharged state, over a given time period. Aside from that, a look at the datasheet, or an educated guess based on battery chemistry is required.

Standard NiMH Rechargeables for example, have a high discharge rate, noted of up to 4% per day, or 40% per month, when stored at room temperature. It is higher at higher temperatures, and lower at lower temperatures. That means a regular 2300mAh 1.2V AA sized NiMH cell discharges in the mid to high double digit milliamp range per day!! Alkaline batteries on the other hand, have a 5 year shelf life. The same 2300mAh AA battery but with Alkaline chemistry discharges at 1mA or less per day.