The time needed to replace the board underneath your sink will vary heavily from one project to another.
However, the average time taken by most homeowners ranges between three and four hours. This can include removing any damaged fixtures and installing the new ones for good measure.
If you are replacing particle board with wood, make sure that you have a decent amount of time. Wood is much harder to work with as compared to other materials and can take much longer than if the material were something else.