Does anyone know why this is on hiatus? And how long it has been on hiatus?

Supposedly, the creative team are getting divorced and as a consequence, splitting as a team as well, though the writer is looking for a new artist to continue the series. But I haven't found any references to an official annoucement, only to a 2chan post. So I'm not entirely convinced just jet.

[url]http://zepy.momotato.com/2008/06/09/he-is-my-master-manga-authors-divorce/[/url]

Shouldn't we mark this as "Cancelled" at this point?

Frankly said I don't see this getting continued. The Japanese Wikipedia says it's "de facto cancelled", and I agree. Neither his blog, twitter, or the 2ch thread he posts in with trip point at new chapters ever getting released.
There is a limit to what you can still call "Hiatus".

I agree. It seems like this manga is over. It has been on hiatus for way too long already. Just pack and leave because it doesn't seem like it will continue anymore. I'm also wondering about the manga Nana too.