How long after long flight is there risk of DVT if you go walking right away?

Walk! The risk of DVT is while you are motionless on the plane. Getting up and moving is the best possible thing you can do to prevent dvt. Perhaps you are talking about a pulmonary embolism. That would mean when the clot is dislogded. Most pulmonary embolisms to happen early in the course of an untreated dvt. The longer it's in place the more less likely it is to embolism.
Dvt. air travel can cause a dvt, and as such use of support or compression during travel very helpful also baby aspirin beneficial the time frame of the dvt is usually within first day or so after the travel leg discomfort or swelling should trigger evaluation.