That is totally up to the individual. How hard are you willing to work at it? Seriously, take a look at Nat, Dan, Fern and others on this board that commited to themselves to work out and get fit. They have super charged their metobolism in less than 6 months.

Me personally I have a hard time keeping up the exercise for more than a couple of weeks because I grow bored with it.

Just like staying on this plan during stalls, it takes commitment to an exercise program.