For he longed after you all, and was full of heaviness, because that ye had heard that he had been sick.
Note 5 at Php 2:26: Paul's statements about Epaphroditus give us clues as to how long Paul had been imprisoned in Rome at the time of the writing of this letter.
The Philippians had to hear that Paul had finally reached Rome. In those days, news of Paul's whereabouts was not sent to them by means of telecommunications. An individual had to physically travel from Rome to Philippi and deliver the message. This would have taken a minimum of weeks and possibly a month or more. Then the Philippians had to send Epaphroditus with aid for Paul from Philippi to Rome, and that would again have entailed lengthy travel.
Once Epaphroditus reached Rome, a period of at least two months would have elapsed since Paul had reached Rome. It could easily have been longer. Then Epaphroditus became ill. News of his illness had to travel back to Philippi, and the Philippians' response had to travel back to Rome, for Epaphroditus to know that the Philippians had heard of his sickness.
The shortest time in which these events could have taken place would have been a minimum of three to four months, and it could easily have taken place over a year's time. Paul was imprisoned in Rome for two whole years (Ac 28:30). This letter was written at least months after Paul arrived in Rome and possibly toward the latter part of his Roman imprisonment (see Life for Today Study Bible Notes, Introduction to Philippians, Date and Place of Writing).