CX debate generally goes by this timetable:
1st Affirmative Constructive- 8 minutes
Cross-examination of 1st Aff- 3 minutes
1st Negative Constructive- 8 minutes
Cross-examination of 1st Neg- 3 minutes
2nd Affirmative Constructive- 8 minutes
Cross-examination of 2nd Aff- 3 minutes
2nd Negative Constructive- 8 minutes
Cross-examination of 2nd Neg- 3 minutes
1st Negative Rebuttal- 5 minutes
1st Affirmative Rebuttal- 5 minutes
2nd Negative Rebuttal- 5 minutes
2nd Affirmative Rebuttal- 5 minutes
Debaters use the constructive speeches to build up their case and attack the other side. They use the rebuttals to explain to the judge why they should win, based on what has already been discussed. Like Foxy Penguin said, nothing is completely set in stone. However, there are a few "rules" CX debaters follow. In cross-examination, the cross-examiner may only ask questions, and no new evidence is allowed in the rebuttals (In general. There may be exceptions, although I haven't encountered them).
For an example of a policy debate plan, refer to 2001-2002 Debate Topic