If this was a filmed interview, it is likely just a mistake for
People ask, and have been asking for twenty years, ...
which is perfectly good American English. When people are being interviewed on camera (and I have some experience with that), they may start to say one thing and realize that they want to qualify or expand their original thought. What then frequently happens is that neither the original nor the revised thought gets expressed correctly.
People ask for twenty years
is not idiomatic modern, American English.
People have asked for twenty years
People have been asking for twenty years
If you want to stress that they continue to ask today with high frequency you might say
People ask, and have [asked/been asking] for twenty years