How long will it take an Australian to pick up an American accent?

Okay so I'm from australia and I just wanted to know if I move to America when I'm let's say 19 (17 now) how long will it take me to pick up the American accent? Oh and I've heard that if you like the accent it helps quicken the 'process'. I've ALWAYS admired and wanted an to speak like Americans. So approximately .. How long?? :/