We should not expect to discover the exact motives and methods of those involved with the birth of: "The Gospel according to Mark". But it is not irrational to presume they would have acted as people would do in the same situation today.
Luke's arrival in Rome, with his gospel, must have electrified the Christians in Rome and have become the centre of discussions.
Matthew had written for the Jews- to proclaim the fulfillment of the Hebrew prophecies. Luke's Gospel was for the Gentiles. Extra information and any apparent contradictions or discrepancies would have raised questions. Also the question would have arisen as to the suitability of reading Luke's Gospel in the churches.
There was a need for Peter to comment on the two documents. Orchard has suggested that Paul would have especially wanted Luke's work to be accepted by Peter, so it could be used in his Gentile churches.
Another consideration could have been that when an evangelist entered a town, it was customary to first visit the synagogue. Using Matthew's Gospel, a nucleus of Jewish believers would be formed and these used to preach to the Gentiles. But Paul was planning to go to Spain where few synagogues existed. An endorsement of Luke's Gospel would enable Paul to use it to open his preaching.
This situation would have led to the decision to hold a day-long conference where Peter could address the community. The task before Peter would not have been easy. The two documents would need to be closely examined, line by line, by Peter, Luke, Paul, Mark and probably others.
Orchard suggested that Peter's commentary consisted of five half-hour talks. If we accept this suggestion we may presume there were 15 or 30 minute breaks between each talk.
A longer break at midday would allow time for lunch, rest, prayer and informal discussion. Following the end of the fifth talk, a time would have been allowed for the audience to ask questions. Starting at say 10 am the conference would have concluded at about 6 pm.
from chapter 3, The Clementine Gospel Tradition