Under U.S. law, you are expected to apply for asylum either within one year of your last U.S. entry or one year after your valid visa status expires, unless you can show "extraordinary circumstances" or a similar exception.
If you have filed an affirmative application for asylum in the U.S. (meaning one where you are in the U.S. but not in removal or deportation proceedings), you may find yourself waiting for years for an interview date and decision.
As someone legally in the U.S. on a valid visa, how to handle fear of returning to your home country can be tricky. There is a one-year deadline between entering the U.S. and applying for asylum; but USCIS is often willing to make an exception for people who were in lawful status.
The idea behind a credible fear interview is that noncitizens can, instead of being sent straight home, apply for asylum or withholding of removal if they are able to establish a credible (believable) fear of persecution or torture if returned to their home country.