For the record, in Japan the minimum notice period (for both sides) is 2 weeks by law. However, no company will fire you under such a short notice unless you do something egregious like punching your boss.

If you're leaving, most companies will expect at least a 1-month notice. Two months is common. Even if you're changing jobs that's not too long because the other company (if it's a Japanese company) will understand and actually expect it. On that 1~2 month period you're expected to hand over your tasks to a colleague.

If you're leaving for a reason other than changing to another company (like Patrick), then it's not uncommon to give a longer notice. For instance, many women still leave their jobs after marrying, or after getting pregnant, or when their husbands get transferred etc.

I've heard of key technical staff having a 3-month notice written out in their contract. Nine months is ridiculously long no matter what.

Wow. Where I am, the notice is 2-3 months and contractual so there is no negotiating it.

Personally, I would tell the person "Go ahead and go now, and we'll just continue to pay your salary to the end of the contract". If the person is really unhappy with the place, having them around bringing everyone down is going to cost a lot more than paying someone 2 months to not be there IMO.