Q:  After how many hours of air travel, it usually becomes a risk factor for pulmonary embolism (PE)?

Answer: After about 4 hours

Hard to predict but, The PIOPED II study described travel of 4 hours or more in the previous month as a one risk factor for PE. 


Reference:

Stein PD, Beemath A, Matta F, Weg JG, Yusen RD, Hales CA, et al. Clinical characteristics of patients with acute pulmonary embolism: data from PIOPED II. Am J Med. Oct 2007;120(10):871-9.