ORIGINAL: snuts
When the nominal voltage is 6.6V -Fly- It will stay at this for quite some time, but once it shows below 6.6 look at a recharge. That said, you will get a LOT of flying in between charges.
-Snuts-
I agree with this. The question is about the
minimum voltage with which you can
safely fly. It is not the same as "what should I run my batteries down to before I recharge". RadicalRC suggests never letting the voltage get below 3.0 volts per cell, which would be 6 volts on your two cell battery. Because you are running a lithium battery which is very efficient, flying the plane at 6.6 volts for one flight will NOT bring the battery to a dangerous level. I believe this is the lowest point with which you can safely fly the plane AND not damage the battery.
That being said, a fully charged 2600mah lithium battery should never get close to that point after one day flying, no matter how much you fly the plane. I don't even think it would get close to that after two full days of flying. So, while 6.6 could be the absolute minimum voltage to fly safely, you really shouldn't even be getting close to that number.
http://www.radicalrc.com/lipolyinstructions2.html
7. DO NOT discharge Lithium type cells below 3 volts per cell. Under 3 volts chemical reactions start that ruin the cells capability to store a future charge. Lithium type cells allow us to extend our projects to previously unknown capabilities. However, they must be treated with respect, care and caution at all times.