Some info on duration, force and times per day:

https://www.google.com/search?q=cerv...ion+guidelines

http://www.djoglobal.com/sites/defau...tion_Sheet.pdf

http://www.jospt.org/doi/pdf/10.2519....2001.31.4.207

"Others17 have found that a three-week course of mechanical cervical traction along with a hot pack and exercise program reduced complaints of numbness significantly more than hot pack and exercise alone." ... so you were asking how long for improvement. One study tested 3 weeks of treatment. I don't know that they checked at early timeframes to see exactly when the benefits kicked in.