It wasn't too bad, once I figured out the right method and used the right tools. The thing is that there are no shortcuts, you have to do it in steps. I also didn't want to go crazy and do too good of a job. The goal was to make it look polished aluminum and not get a mirror finish, which would be hard to maintain, and the tank isn't perfect to begin with...
To be honest I couldn't tell you how long it took exactly... maybe a day? It could be done much faster now that I know how to go about it though.