RE: Blade CX 2 charge
You can fly for around a max of 8-9 minutes on a fully charged pack. Check the voltage of a fully charged pack, it should read 8.4v.
As to how you know.... most people time it, on a FULLY charged battery, time how long you fly (really fly not just have the battery plugged in) and fly for around 5 minutes, stop and read the voltage (use the balance connector so you don't have to unplug the main connector), repeat until you get to 6.6v. See how long you were flying. Do this a few times so you really know what the time is then use that time as a max.
Don't push your batteries, an extra minute of flying time per charge is not worth destroying a battery. Just buy extras so that you can have one (or more) charging while you're flying.