There is no minimum time because what you want to do is impossible. 'Nanopowder' is an oxymoron - there are no free, independent, discrete particles < 100 nm in a powder.
First of all, it is possible to convert micro into nano powder through chemical reactions only which is very complex phenomenon that once i tried but failed.
and ball milling is not for conversion of particle into nano-particle (it is for mixing only)
With regard to the previous answer: ball-milling technologies for conversion micro to nanoparticles exist. For example this is one of the pretty broadly used options for deaggregation of detonation diamond to nano-range.