Dulce de leche is rich creamy caramel sauce - boiling an un opened can of condensed milk for 3 hours creates beautiful caramel. It is nicer than the ready made one which always tastes wrong to me.
I think from memory in the slow cooker it took 5 hours on high and 7 on low but not 100% sure from memory. Once cooked you can store un-opened.
Always keep the can covered with water while simmering - to prevent the can from rattling in the water (which can be annoying when you consider that it'll need to be in there for several hours) put a rag under the can.