For some reason, probably in the midst of his work at Corinth, Paul took on himself the Nazarite vow. This could only be terminated by his going up to Jerusalem to offer up the hair which till then was to be left uncut. But it seems to have been allowable for persons at a distance to cut the hair, which was to be brought up to Jerusalem, where the ceremony was completed. This Paul did at Cenchrea just before setting out on his voyage into Syria (Acts 18:18).
On another occasion (Acts 21:23-26), at the feast of Pentecost, Paul took on himself again the Nazarite vow.
"The ceremonies involved took a longer time than Paul had at his disposal, but the law permitted a man to share the vow if he could find companions who had gone through the prescribed ceremonies, and who permitted him to join their company. This permission was commonly granted if the new comer paid all the fees required from the whole company (fee to the Levite for cutting the hair and fees for sacrifices), and finished the vow along with the others.
Four Jewish Christians were performing the vow, and would admit Paul to their company, provided he paid their expenses. Paul consented, paid the charges, and when the last seven days of the vow began he went with them to live in the temple, giving the usual notice to the priests that he had joined in regular fashion, was a sharer with the four men, and that his vow would end with theirs.
Nazarites retired to the temple during the last period of seven days, because they could be secure there against any accidental defilement" (Lindsay's Acts).
As to the duration of a Nazarite's vow, every one was left at liberty to fix his own time. There is mention made in Scripture of only three who were Nazarites for life: Samson, Samuel, and John the Baptist (Judg. 13:4,5; 1 Sam. 1:11; Luke 1:15). (See RECHABITES.)
Therefore if Paul himself took on the nazzerite vow from time to time, then why would he tell Nazzerites to cut their hair?
This shows that Paul was not speaking to Nazzerites, but men who were simply growing their hair as if they were Nazzerites.