I have tried to look up how long St. Peter served as Bishop of Rome but have not as yet been able to find anything. Hopefully either I can find something or else someone else can post up the information which shows how long he served in Antioch and how long he served in Rome.
This is the understanding of Pope Shenouda and the Coptic Orthodox Church.
Peter was in Rome only for two years before his death.
Go to Chapter IV. The Issue of the PRIMACY OF PETER:http://web.archive.org/web/20050827230121/http://www.stmark-la.com/book.html
7. Facts about Peter:
1. 44 AD.............Peter was imprisoned in Jerusalem at 44 AD so how was
he present in Rome at that time?!
2. 45 AD.............Clodius Caesar exiled all the Jews and the Christians
from Rome at 45 AD, and the book of Acts made reference to this event (Acts
18:2). So it is again impossible for Peter to be in Rome then.
3. 50 AD..............In 50 AD, he attended the apostles council in
Jerusalem, so it was impossible for him to be in Rome then.
4. 57-58 AD.........St. Paul wrote to the Romans in 57-58 AD asking to be
given a chance to reach them and teach them about God. This is a proof that
Peter did not preach the Romans in Rome, otherwise Paul wouldn't have asked
to be given a chance to go.
5. 58 AD...............In 58 AD when Paul sent his epistle to Rome, he
greeted 20 people, and 2 families, and the name of Peter was not among them
which means that he (Peter ) was not there at that time.
6. 60 AD..............When St. Paul reached Rome at 60 AD, the Book did not
tell us that he met with Peter, but rather Paul met the leaders of the
Jews.. thus proving that Peter did not preach them with the Lord Jesus.
7. 62-63 AD.........St. Paul stayed in Rome for two years after preaching
the Romans, (62/63 AD) meaning that if Peter reached Rome then, the church
of Rome was founded, established and was strong by the works of the Holy
Spirit and Paul.
8. 65 AD..............Therefore we acknowledge what Origen said, that, St.
Peter came to Rome before he died, about 65 AD, to chase Simon the sorcerer,
who offered money to him (Peter) and John for the power of the Holy Spirit
(Acts 8:9-24), and Peter was crucified there and died.