They'll discharge themselves to 50% if left alone for 10 days, but it will take a few days to do so once the process starts, so about 2 weeks in total, IIRC. If you want to fly your drone right up until *you* fly, then that won't work unless you manage your flying to drain them all to your preferred level on the last day's flying. Alternatively, if you have a FlyMore kit, you'll have a little adapter that lets you charge USB devices from one - you can use this to charge your phones, tablets, etc. and drain the batteries that way. Using them to charge a USB powerbank will take a particularly big bite out of their charge.
Personally though, I wouldn't worry about it too much. Sure, if anything happens having less charge in the battery will reduce the energy involved and give people more time to deal with it, so if you can discharge them that's all for the good. However, as long as a battery is well packed against knocks and the terminals can't easily be shorted, then then chances of it cooking off in the first place are near zero. You only hear about battery incidents at all because it's an exceptional event; people fly with LiPos all the time yet you only hear about incidents every few months or so, and more often than not it's a knock-off Li-Ion in a phone anyway.
