~100 years

The Hoover dam is a bit of an outlier because it is such a large project. The larger the dam the longer the lifespan usually. If cracks in concrete are found they will attempt to fix them by draining the water level for a time. The Hoover dam also contains no rebar which helps a lot for longevity.

Without human intervention the dam would collapse in 3-5 years. With regular service to the turbines but no structural work I’d guess ~50 more years.

But again, the Hoover dam is a huge outlier. We have over 80,000 dams in the US. Most of these dams will be brought to the edge of their lifespan, torn down, and rebuilt rather than repaired or retrofitted.

Civil engineer here, age is definitely a factor. Time eats through concrete like any other material. Water will never flow “over” a dam, there are safeties in place, if it does you have much bigger problems.

Also “beems” in building are made of multiple materials, one being concrete. I appreciate the confidence, though.