I agree with the others above who believe the information is not being deleted. IANAL, but from a legal perspective, I believe it doesn't matter if the NICS data is kept for one minute or one hundred millennia since the very requirement that the information be collected from a firearms buyer is in violation of the Fourth and Fifth Amendments.
One of the predicating notions that this country was founded upon is that private information and property not fall into the government's hands without probable cause or a warrant. Simply desiring to procure a firearm does not give the government cause to demand private information if due to nothing else than the existence of the Second Amendment. Simply fearing that a firearm will be used in a crime is no excuse either, since prior restraint is an illegitimate legal tool on its face.
Furthermore, even if the government DID have cause to demand private information, the accused is under no obligation to aide in its production. If the government has a case, let it be made, but THEY are the ones that must do the foot work, not the person under threat of fine or imprisonment.