Donal,
I hate to say it but your battery is shot. If you google how to bring a LiPo back to life, you will find tons of information on people who have brought ones back from the dead. You have to have some specialized equipment (adjustable power supplies). But it is MOST DANGEROUS. An over discharged battery is prone to catch fire when a recharged is attempted, I would only do so outside.
YES it can be done.
NO I do not know how long it would last if you do resurrect it from the dead.
Just be careful.