If you wait until January you should be gold through March of 2013. You would get it from the remainder of the year you quailify (2011) and all of the following year (2012) through march of the next year.
If you think you will have $20K in spending in 2011 or qualify the "traditional" way for gold in 2011 and have any hilton stays planned before year end, I would upgrade imediately so you can enjoy the gold benefits right away and get the bonus points on your hilton stays.