Since there is no scriptural information regarding there being a gap in between the time that the church is gathered and the beginning of the seven years, it would just be assumption on anyone's part. However, I think that the gathering of the church, where millions of people are removed from the earth would be a perfect time for the revealing of the antichrist and his establishing if that seven years. I've always entertained the idea that after the church has been gathered, that the antichrist would be revealed and would offer his explanation as to what happened to us, an unfavorable one of course.
If there is a gap, I don't believe that it would be to big of one. Who knows! All scripture does says is that "the day of the Lord" which is the wrath of God, will not come until apostasy takes place and the man of lawlessness is revealed. How much time there is between the time we are gathered until the antichrist is revealed is anyone's guess. Once the church is gone and the Holy Spirit is no longer restraining the full force of sin and the man of sin from being revealed, this earth is going to get really, really bad, to say the least.