How long is the k-3 visa good for? In other words, if my husband had to stay in Egypt for a while after getting the visa, how long before it's no good?

K-3 visa is a 2 year multi entry visa, so it is good for 2 years.

Thanks

He must enter the US within the two years of visa issue. Then the two year clock begins again, during which time he may come and go as he pleases for two years from entry date. During the two years from entry date he adjusts status to permanent residence. If AOS takes too long the validity of the visa can be extended.