This is a difficult question to answer, as there are many factors that can affect how long a Duramax engine will last. However, in general, most experts agree that a 6.6 Duramax engine should last between 250,000 and 300,000 miles with proper maintenance. With that said, it is not uncommon for some engines to last much longer than this (up to 400,000 miles or more), while others may only last 150,000 miles or less. Ultimately, it all comes down to how well the engine is cared for and maintained throughout its lifetime.