In London, I'm currently on a 2 month notice, however that has always seemed unusual to agents and people I interviewed with. Also, that's a fixed period for senior and junior position - in other companies I worked for, there was always a special clause like "4 weeks + 1 week/year after the Xth year, capped at Y weeks"

That must be very dependent on your company sector and market. Also, notice is generally reciprocal, so a bigger one is generally considered a benefit - as practically, the company has to pay you when they terminate you, but you can almost always negotiate a much shorter notice when you resign.