I think the flood protection goes hand in hand with the life of the plant and with the risk that it has once it stops operating, that is why the nuclear power plants have a high flood period protection.

So the average of most power plants nowadays, except the nuclear ones, is 100 years.

There is no correct answer to this question. It depends on how much risk the stakeholders running the different types of power plants are willing to take and/or if there are any regulations in place that require certain level of protection to be put in place.

For example, hydropower is often generated via the construction of a large dam. The general safety standard for high hazard dams, (i.e. dams with people living downstream of them) is that they should safely pass the

Probable Maximum Flood (PMF). The PMF is is the theoretically largest flood resulting from a combination of the most severe meteorological and hydrological conditions that could conceivably occur in the catchment upstream of the dam. This paper entitles "Risk analysis for dam safety evaluation gives some details (see: https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.179.2568&rep=rep1&type=pdf).

With regards to nuclear power plants these regulatory guides from the US published in 1975 and 1976 give some idea as to what was planned for at that time in terms of floods (see: https://www.nrc.gov/docs/ML1335/ML13350A290.pdf and https://www.nrc.gov/docs/ML0037/ML003740308.pdf). For nuclear power plant safety risk assessments, flood estimates are needed for return periods of up to 1 million

years (i.e. exceedance probability of 1 in a million) or even the PMF and Probable Maximum Precipitation (PMP) for surface water flooding. This site maybe of interest: https://allthingsnuclear.org/dlochbaum/flooding-at-the-nuclear-plant. In India it looks like they use the PMF and PMP see: https://www.aerb.gov.in/storage/uploads/documents/regdoc6pLGP.pdf

This updated document for the US is also worth looking at: https://www.nrc.gov/docs/ML1132/ML11321A195.pdf