Whenever it feels right. There is no hard answer and it depends on the match-up, your resources, your opponent's habits and your own skill level with the forms. Whatever you get to work for yourself.
For example, I spend most of my time in the Hakumen match-up in Luna mode now. I'm pretty sure almost every other Nu on here uses Dia mode. Despite it seeming like Dia would be the better mode, I can't get it to work properly against him like you all can so I just gave up trying to use it. Trial and error your way to victory.