How long will a drug-eluted heart stent last before a new one is needed again?

6 months. The highest risk of stent renarrowing is in the first 6 months. If you get through the first 6 months without a problem, the rate of renarrowing falls dramatically. Overall, the rate of renarrowing for medicated stents is 5% in for an medicated stents 20%.
Your whole life. Stents are for life. They become embedded in the wall of an artery. If a blockage develops in the stent there are a variety of methods for dealing with this. It would never be replaced... One has to work around it.