A footnote at the Christian Classics Ethereal Library (in Eusebius, Bk II, Ch 15) says:
*Upon the historic truth of Peter’s visit to Rome…Although we may accept it as certain that he did visit Rome, and that he met his death there, it is no less certain that he did not reach there until late in the reign of Nero. The tradition that he was for twenty-five years bishop of Rome is first recorded by Jerome (de vir. ill. c. 1), and since his time has been almost universally accepted in the Roman Catholic Church, though in recent years many more candid scholars of that communion **acknowledge that so long an episcopate there is a fiction. ***
Out of curiosity, how long was he there?