How long do boss corpses last

Q u o t e:
im asking because i want to get the omen achievement on my dead server on two chars

Unfortunately, I'm unable to provide a specific time frame. Your best bet is to ensure that both characters are in a position to take advantage of Omen's demise in rapid succession.

Q u o t e:

an why is that.

Because I don't know =). And providing specific mechanical information of that nature isn't in my purview - typically we aren't in a position to discuss game mechanics in that kind of detail.

This happens to be one of those situations. Fortunately, options exist for your to benefit from the raiding activity of other players if you plan ahead.