When did the mitochondia and the chloroplast become nuclei?

They didn't. I think you have misinterpreted something here. Are you
asking about mt and cp DNA, or are you interested in the transfer of
their genes to the cell's nucleus?

If you mean become organelles than this question is simple. The
earliest cells, prokaryotes (bateria, archaea) performed their energy
making tasks through glycolysis, a simple conversion of glucose to
pyruvate with a relatively small ATP yield. However as evolution took
its course, eukaryotes evolved and worked mostly the same way. The
endosymbiotic theory states that one of these early eukaryotes
consumed a prokaryote that was able to further process the products of
glycolysis and produce more energy. The same is true of chloroplasts
and flagellae.

Evolution continued and these prokaryotes developed on their own
(mitochondira have seperate DNA from the host cell) additions and the
process we know as the Krebs Cycle was formed.

http://www.ruf.rice.edu/~bioslabs/studies/mitochondria/mitorigin.html