Any blood clots including blood clots in your leg are dangerous and are considered a medical emergency and if you don't get the blood clot in your leg or anywhere else in your body cleared then you can die.
You could die the same day or you could die a month or so later as it just depends on how severely clotted the leg is.
Blood needs to flow freely and unrestricted through your body including your legs and if not it can lead to a heart attack, stroke and even death.
You may need to have a leg stent put in to keep your blood from blocking up in your legs veins if it's severe enough.
Other times they may just need to clear the blood clog surgically.
Either way the blood clot needs medical attention if you want to survive.