How long?

So I started playing this game a few years ago on my son's DS. He had no interest in the game at the time. Then he broke his DS. Got him another one a year later as I didn't want him thining he could break anything and I would just replace it. He played over my file which was fine as it was his game but then got bored with it. I was thinking of starting it again but have a question. About how long does this take to get through? Not collecting everything but just beating the main story? What is a realistic time range? I like to beat a game before I start something else which is why I ask, so I'm curious as to how long I'll be playing this if I start it up again from the beginning.