I'd like to say: just give it a try ! Cu(+II) forms deep blue complexes in ammonia, but you don't need high concentrations of copper to observe them. So the appearence of the deep blue colour does not mean that all CuO was dissolved. It might also depend on the shape (powder or bulk) and on thermal history of your oxide. It is much easier to dissolve when freshly formed than after a long recrystallisation at high temperature.

In all cases, you should try with very concentrated ammonia.

Alain

It is also better to have an idea of Pourbiox Diagram, that helps to get more details of CuO dissolution in ammonia.