Most Priority Mail takes about 2-3 days depending on the distance. In this case, since the distance from California to West Virginia would be nearly crossing the whole of United States, you can expect it to arrive at around 3 days.
United States Postal Service can not guarentee standard mail to arrive by any specific time. It is estimated that standard mail will arrive 2-9 days. If you require letter to arrive earlier you can send it Priority Mail, which will arrive within 2 days in most cases.