The short answer according to longstanding FTC guidance is 6 months from launch, with a little extra time built in if you do a limited test launch first.  This rule has been on the books a long time but not actively enforced.  The FTC likely has better things to do with its resources than protecting those who may be moved to purchase based on a product being new to market when it has been around awhile.

But "new" claims are often examined at the NAD.  They recently brought a monitoring case against Tiffany & Co.  A monitoring case is one NAD brings itself based on its review of ads not based on a challenger or consumer starting an action.  Here the NAD looked at claims for Rubedo jewelry that Tiffany had created a "new jeweler's metal" combining primarily gold, silver and copper for a pink toned metal.  The NAD inquired as to whether this really was new and even if so if it implied a new element in the periodic table.  Tiffany's had applied to patent its new metal and had advertised as new for less than 6 months.  Not messing around, Tiffany submitted an expert report from an MIT professor opining the metal was a new precious metal alloy development.  NAD concluded the product was in fact "new".  And answered its own question that consumers would not reasonably interpret the claim that a new element had been found but that "new" would be understood as a new metal alloy.  This is one of the rare monitoring cases where NAD rendered a decision for the advertiser.  NAD did appreciate that Tiffany had already discontinued a different claim that Rubedo contained the riches of gold and the brilliance of silver as it was concerned this might overstate its value.  NAD administratively closed this portion of the case.

So while many do not view being challenged by the NAD with rose colored glasses, the decision was certainly in the pink (or the Rubedo) for Tiffany.