My partner (m, 48) drove 12 hours with only one break in between after 6 hours. is there a risk of dvt in such situations? what to watch out for and how long does it take to dvt to occur after risky situations?

If it's just one time of long driving with one break after 6 hours and he is also young, the DVT risk is minimal, you should not worry too much. However, if DVT occurs it will happen during that 12 hours of driving when the venous return is sluggish. Symptoms of DVT are asymmetric leg swelling, dull aching of the calf and severe chest pain with shortness breath when the clot moves to the lungs.