The King of France during this period was Charles VI. Although there were rumours in Paris since early December, which prompted the King to send messengers to Hungary to seek information. The first reliable news of the defeat to reach Charles VI was when Jacques de Helly who was one of the knights who fought at the battle of Nicopolis arrived in Paris to tell Charles of the defeat.
Jacques had been released by the Ottomans so he could inform France about the defeat and to deliver a message about the ransom that the Ottomans demanded in order to release the other prominent French prisoners.
The battle of Nicopolis happened on the 25th of September. Jacques arrived in Paris on the 25th of December, so it took precisely 3 months for Charles to receive evidence of the defeat, and it took a little over 2 months for Charles to hear rumours about the defeat.