The Prophet ﷺ never fasted a full month except the month of Ramadan, and he used to fast till one could say, "By Allah, he will never stop fasting," and he would abandon fasting till one would say, "By Allah, he will never fast."
مَا صَامَ النَّبِيُّ ﷺ شَهْرًا كَامِلاً قَطُّ غَيْرَ رَمَضَانَ وَيَصُومُ حَتَّى يَقُولَ الْقَائِلُ لاَ وَاللَّهِ لاَ يُفْطِرُ وَيُفْطِرُ حَتَّى يَقُولَ الْقَائِلُ لاَ وَاللَّهِ لاَ يَصُومُ