As I understand, for US dealerships a build slot only becomes available for an Aventador only when that dealership has sold 2 new Gallardos. It entirely depends on how fast a dealership is able to sell 2 Gallardos. I've heard many cases of owners having to wait similarly as in your situation.
I know it's only wishful thinking, but if I had a boat-load of money, I would easily order two LP-550's drive one of them for the meantime, sell the other one and wait 3-6 months for my Aventador to come in presuming a build date would open up rather quickly after buying those LP-550s.
Sure beats waiting for a year or more, IMHO.