Depends on altitude, whether or not you are under water, other sounds such as wind or rain. Are you in space? a canyon? many variables. but, on a flat, quiet area I would say about 1/2 mile max for a normal person. A person whistling may make it a mile.
Hmm, Well if i had to guess i would say around 1 mile. I know have prolly screamed around a half mile and im guessing im not the strongest screamer out there....very weird question tho