I'm not going to speculate as to how long it will take, there's obviously too many variables. Two of the guys I've lifted with benched over 400 after about 4-5 years of lifting, one of em is closing in on 500 after about 8 years of lifting. Me, I plan on hitting 400 at my first meet in October, where I'll have been lifting for about 8 years. Had I known what the hell I was doing, I probly would have pressed 400 a year or three ago.
Anyways, what I would really caution here, is not lifting too heavy. Those two guys I mentioned? 6-7 years ago, they were warming up with 135, right? I didn't think much of it, and did the same. 135x10 when your max isn't much over 225, is a work set.
Get this guy on a good program, and I wouldn't be overly surprised if it took a single year. If he putzs around with a bad program, or doesn't follow the good one, doesn't warm up properly, overtrains, etc, he might be able to still get there, but it might take a decade instead.