“For patients undergoing major orthopedic surgery, the American College of Chest Physicians recommends outpatient thromboprophylaxis for a duration of up to 35 days. Older recommendations for 10–14 days of prophylaxis were based on studies performed when this was the usual hospital stay. This is still recommended as the minimum length for prophylaxis, but a longer period of time is preferred.” ABFM
The following Cochrane review didn’t find any difference between the short and long duration of treatment. Read it.
http://www.cochrane.org/CD004179/PVD_anticoagulants-taken-longer-periods-prevent-deep-vein-thrombosis-or-pulmonary-embolism-after-hip-or