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By finished, I assume you mean beating the E4, right? Hmm, I think I got the game on a Monday, and beat the E4 by Friday... And I played it for less than 15 hours. It's been like, a year and a half though, so it's hard to remember.
Platinum, well, I've put over ten hours on it already, and I don't even have the first badge. XD I was EV training my Chimchar for fun.
Somewhere around 14 to 18 hours. I remember that I finished it really quickly compared to most people, I was done with the E4 in about 3 days. I guess I could check on my game--oh look, shiny!
I enjoy restarting my game FAR too often ( I think about 10 or 11 times now) but the quickest amount of time I finished was 10.5 hours whilst the longest was about 35 hours (making UU/NU parties effective is difficult)