Normally, post-production only starts when principal photography ends, but they may overlap. The bulk of post-production consists of reviewing the footage and assembling the movie – editing. The time editing takes to complete is directly proportional to the amount of footage recorded and how good camera notes are.
An infamous example is the case of Apocalypse Now. Shooting indiscriminately, sometimes with eight cameras rolling, Francis Ford Coppola ended principal photography with tons of footage that were good for nothing. His editor, Walter Murch, who started working before Coppola was done, spent two years in the editing room figuring out how to piece this epic together. Camera notes help in the sense that they tell the editor where to look and what to look for.