A lot will depend on you and how your health is. Have you taken steps to slow down any of the heart disease? That should be your first step. How do you feel? What treatment was implemented by your doctors? How well does your heart function with these blockages? Where are these blockages?
That depends on many circumstances, but even then, a time limit couldn't be accurately applied. Although many times arteries are fully blocked in patients, small natural bypasses called collaterals open and feed the area deprived of blood. IF, the LAD (left anterior descending) and Lcx (circumflex) were totally blocked at the top with no collateral development, I doubt if anyone would survive more than a couple of minutes. The left ventricle just wouldn't get enough oxygen to keep the muscle alive.