With modern lithium batteries, you do not have completely discharge the battery--in fact, it's risky to do that, because if the battery actually reaches true zero, it's irreversibly discharged.
As a general rule, I start recharging when I reach 30-40%.
Some Samsungs have an error where you need to disconnect the battery to get it to charge again if you let it run down. My 7.0 LTE has been run flat about 15-20 times and not done this yet which is great.
If I commit to the Flex I'm not sure if I can even MAKE the charge hit 20% or less before I get home :3