Corns being crepuscular in nature (active at dusk and dawn), I would suspect this is the time of day when you'd be most likely to find one out hunting in the field.
Of course, this is going to vary depending upon the type of snake you are looking for...some are nocturnal, some are diurnal (Garters for example), some are crepuscular, and some mix it up depending upon season, temperatures, etc.
note - the above is based upon online research, not actual field experience