Promotion tends to come faster when an army either expands quickly in wartime or because of casualties incurred, or both.

For example:

-Berthier went from LtCol in 1789 to general of brigade three years later.

-Davout was first commissioned in 1789 and was a general of brigade five years later.

-Grouchy had retired from the service in 1787 but volunteered in 1791 and was a general of brigade the next year.

-Lannes was a lieutenant in 1792; made colonel in 1795 and was a general of brigade two years later.

-Ney was a sergeant-major in 1792 and made general of brigade four years later.

-Massena was a retired sergeant-major in 1789. He volunteered for service in 1792 and was a general of division the next year.