If you are discharging to storage then your charger will almost certainly have a special 'storage' setting. The storage setting takes the battery to optimum voltage for storage which is in the 3.8-3.85V range. It will automatically charge or discharge to that voltage depending on the state of charge of the battery that's connected. You wouldn't want to fully discharge a battery unless you were disposing of it at the end of it's life.
As for 'how many amps'.. just set your charger to the maximum discharge current possible in the programming. It wont discharge at that current anyway, most chargers have rather pathetic discharge power rating, yours is no different at 5W. This means that it doesnt matter what current in Amps you set, the actual discharge current that the charger will work at is limited by the charger's 5W discharge power limit. For instance on a 6S battery the initial discharge current would only be 0.2A (5W/25V = 0.2A)... So if discharging larger batteries it takes absolutely ages!
High end chargers are a bit better with discharge power up to 100W in some cases but most people who use larger batteries rig up some sort of discharger that can handle more power.