Following are some of the Insight or Vipassana Meditation Centers around the world that hold long term silent retreats:

Blue Mountains Insight Meditation Centre, (Australia) Website
Gaia House Insight Meditation Center (England) Website
Insight Meditation Society (Mass) Website
Spirit Rock Insight Meditation Center (California) website
Dharmagiri Insight Meditation Centre (South Africa) website

A more detailed list of long term practice centers and other meeting places by Christopher Titmuss can be found here.