Cajeta is made with goat's milk...
by simmering the the condensed milk you would get something closer to dulce de leche, but even then,not the real thing...
to get real cajeta, follow thelink above, hat's a pretty good recipe, but use only goat's milk - the recipe says an hour of stirring, but it can take more, patience and you will have a great cajeta!
of course, it may just be easierto go buy a jar!