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16" unless it is an NFA registered SBR.
ATF Online - Firearms - National Firearms Act (NFA)
The NFA was originally enacted in 1934. Similar to the current NFA, the original Act imposed a tax on the making and transfer of firearms defined by the Act, as well as a special (occupational) tax on persons and entities engaged in the business of importing, manufacturing, and dealing in NFA firearms. The law also required the registration of all NFA firearms with the Secretary of the Treasury. Firearms subject to the 1934 Act included shotguns and rifles having barrels less than 18 inches in length, certain firearms described as "any other weapons," machineguns, and firearm mufflers and silencers.
Burt is absolutely correct. Here is a link to the ATF site detailing what defines an SBR & SBS.
"A firearm that meets the definition of a SBR consists of a rifle that has a barrel less than 16 inches in length. A SBS consists of a shotgun that has a barrel less than 18 inches in length."
Anything that is 16in or longer is totally legal. Your friend probably confused the rifle and shotgun lengths. It's 18in for shotguns.