How to Do Integration Testing
Integration testing is the testing of the integration of various components that have been unit tested. Integration is expensive and it comes out in the testing. You must include time for this in your estimates and your schedule.
Ideally you should organize a project so that there is not a phase at the end where integration must explicitly take place. It is far better to gradually integrate things as they are completed over the course of the project. If it is unavoidable estimate it carefully.