I found it was best after about 3-4 days, but honestly it's still good to go straight away. no sweetener needed! let me know what you think or if you have any ideas - still want to tweak it a bit!
I highly doubt Tres Leches would work the same way. Dulce De Leche is a very specific flavour. You'd probably be better off with some other kind of toffee/caramel flavour instead. Catalan Cream could work with the caramelised aspect to it. If you try it, let me know how it goes!