He thought that firstly no document or law could be forever relevant, so it needed revisioning occasionally, and the 19 years seems to tie into the idea of each generation taking a new look and either accepting existing laws as still good or making changes.
It may be one of those myths, but I remember that one of the founders initially were proposing the constitution to be rewritten every 10 years.
The French Revolution created an easier method for reforming The Republic and rewriting their constitution.
They enshrined the revolutionary aspects of revolution instead its leaders.
19 years, in a letter from Jefferson to Madison.
To James Madison from Thomas Jefferson, 6 September 1789
He thought that firstly no document or law could be forever relevant, so it needed revisioning occasionally, and the 19 years seems to tie into the idea of each generation taking a new look and either accepting existing laws as still good or making changes.