Duty free allowance does not have any bearing on how long you stay in the country. All that is needed is the normal entry procedure into the USA (custom and immigration). In fact when you leave the USA your passport does not get stamped by US authorities (unless this has changed recently) so there is no real record for your stay period.
Hope this helps.
BS Degree in Engineering.
Math, science and 25 years experience in computer programming and troubleshooting.