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In the song, they do not state exactly how long that takes (to get to the top) - very few bands or groups ever achieve "the top."
However, looking at the group AC/DC itself, they were formed in 1973. They had some success but did not reach #1 in the billboards and finally hit platinum status until 1981 with their "For Those About to Rock we Salute You," album was released. Even though their Back in Black album has sold far more copies, and was released in 1980, it sold those after the release of the About to Rock album.
So, in their case, it took about 8 years to reach the top.