There's no actual limit, other than the natural limitation of human lifespan and the fact that at least in modern times most popes aren't elected until they're at least well into middle age (the average age at coronation of popes since the 1500s has been over 60; John Paul II, who was elected at 58, was considered to be unusually young for the position).
He was pope nearly 25 years.
He was pope for a total of 33 days.
Pope Francis as reigned as pope since March 13, 2013.
"Viva il papa" - long live the pope.
France has the same pope as the rest of the world. Pope Benedict was pope from April 19, 2005, until February 288, 2013.
Pope Joan is a fictional character who never existed.
Pope Francis has been serving as the Pope of the Catholic Church since March 13, 2013. He was elected as the 266th Pope after the resignation of Pope Benedict XVI.
He was pope from August 26, 1978, until September 28, 1978, - 33 days.
The last pope to resign (before Pope Benedict XVI) was Pope Gregory XII who resigned on July 4, 1415.
There are several Pope St. Gregorys so you need to be specific.
The pope remains in office until death unless he resigns.
"Viva il Papa!" - Long live the Pope!