Looking at Boston the last two decades, they have had seasons where they finished 5th in the AL East, but they never have been horrible for a majority of a decade. In 2012 they went 69-93 and placed 5th in the AL East.. In 2013, they end up finishing first in the AL East and winning a World Series.
From 2014-2015, they finished 5th two seasons in a row. Otherwise for the most part they've always been in the thick of things in a tough AL East division.