Any DVT is dangerous, it's just a matter of when. There is no way to predict when you will throw off a embolus and end up with a PE or, if you have a PFO (which most people are unaware they have) you can end up with a stroke. DVTs are not something to mess around with.
Putting a "solid" time to it will be nearly impossible because the clinical course of a DVT is altered by a constellation of concepts. You can look through the literature and probably could make the reasonable conclusion that a DVT is associated with significant long term morbidity regardless of the cause.
One to get you started:
http://www.ncbi.nlm.nih.gov/m/pubmed/9299855/