How long had Paul been encouraging the Greek churches to prepare their offering for Jerusalem?

For a whole year, Paul had been urging the churches in Greece to prepare their collection for the saints in Jerusalem. They had talked about getting these collections ready but had not completed it, so he instructed them to finish it before he arrived, so their giving would not be due to his personal presence.

Les explains that for the last year Paul had been encouraging the churches in Greece to get their offerings ready for the day someone would come and transport that wealth to Jerusalem. A whole year had passed, and they still had not done it. Paul therefore told them to get busy and complete the collection before he arrived, so they would not give merely because of his presence. Les notes this was about Christian giving in general, not a tithe, and he compares it to human nature: many intend to give, but few follow through.

2 Corinthians 8:10

And herein I give my advice: for this is expedient for you, who have begun before, not only to do, but also to be forward a year ago.

Why did Paul have to tell the Corinthians to get busy and finish the collection?

Paul urged the Corinthians to finish because a year had passed and they still had not completed the collection for the saints in Jerusalem. He wanted it done before he arrived so their giving would not be motivated by his personal presence or a sense of obligation.