How long did the monarchy in greece last?

Monarchies didn't last because 1) they depended upon heirs and family relations (lack of children) and, 2) rebellions (the people wanted more say).

Ongoing civil wars western roman empire finally collapsed476 c.e. romulus, the last of theroman  emperors in  the west, was overthrown by the germanic leader odoacer, who became the first barbarian to rule in rome. the order that the  roman empire  had brought towestern  europe for 1000 years was no more.