Others seem to be much better than I am at defining these things. Candidates for a beginning time for the Grace Age have included 1)the ministry of John the Baptist; 2)the ministry of Christ; 3)the death, burial, and resurrection of Christ; 4)the day of Pentecost; 5)the rejection of Stephen; 6)the conversion of Paul; 7)the conversion of Cornelius; 8)the Jerusalem council; 9)the end of the book of Acts. The most common answer I have heard is the day of Pentecost. My approach has been to look at the shift from law to kingdom preaching to grace age as a transition that took place over a period of time with several major events.
In my teaching, I have drawn a time chart with two lines. The time covered is the apostolic age from the resurrection of Christ to the completion of the New Testament--generally from 30AD to 100AD. One line starts at the bottom of the page on the left (30AD) and goes to the top of the page on the right (100AD). This line represents grace age doctrine and practice. The second line begins at the top of the page on the left (30AD) and goes to the bottom of the page on the right (100AD). This line represents the kingdom age doctrine preached by John the Baptist, Jesus, and the apostles initially. The chart looks like a large X. During this period of time, kingdom age doctrine and practice is declining while grace age doctrine and practice is ascending. Where the lines cross, I place the Jerusalem council of Acts 15. The reason I do this is the statement of Peter concerning salvation: "But we believe that through the grace of the Lord Jesus Christ we shall be saved, even as they" (Acts 15:11). In other words, at this point in time, Peter acknowledged that Jews must approach God in the same way that Gentiles do. I do not believe that Bible teachers have given this event the doctrinal importance it really has.
Will the Grace Age last 2,000 years? There are some interesting scriptural comparisons that point to this possibility. The problem is that God makes no such direct statement and this conclusion requires a couple of assumptions. In other words, a 2,000 year Grace Age would fit in some places but no scripture directly teaches this. Let me give you an example. Hosea 6 refers to the return of the Jews from captivity. Consider this verse: Hosea 6:2 After two days will he revive us: in the third day he will raise us up, and we shall live in his sight.
This verse states that God will revive Israel after two days and they will be revived the third day. Many prophecy teachers combine this verse with the teaching of 2 Peter 3:8 which states: "one day is with the Lord as a thousand years, and a thousand years as one day." The two days therefore become the two days since the coming of Christ and the day of revival is the thousand year reign of Christ. Even I think there might be some connection. However, to jump once again to say these "two days" are identical to the Grace Age and therefore all we need to do is identify the beginning of the Grace Age to determine the return time, is to add two more assumptions to the mix.
Hosea 6:2 does not say what the two days are after. In context (v.1), He is dealing with Israel being "torn" and "smitten." It is a bit of a jump to declare absolutely that this refers to the beginning of the Grace Age. Perhaps it refers to the captivity of Israel or of Judah (most of what Hosea says is to the northern kingdom of Israel). If this is so, then the two days probably do not refer to two thousand years, but who said they did. The emphasis in 2 Peter 3:8 is on the meaninglessness of time to God. He also states that a thousand years is as a day but I have never seen anyone try to explain a prophecy by applying that as an interpretation.
My main point is that we have no direct teaching that this age will last exactly 2,000 years. And, although the connections are very interesting, they are not conclusive. We will still have to be ready for Christ to come at any moment. As the Bible clearly teaches in several places, He will come at a time when men do not look for Him. Therefore, we are to be watching and waiting for Him at all times.