It depends on what level of knockdown you are asking about. If you are talking about widespread halting of mRNA translation, it will be faster than than complete ablation of the protein activity. This is because even if translation is stopped, you may have active protein remaining within the cell, and you should wait until it is degraded to truly see the effect of knockdown.

With some genes, I do not see effective knockdown until examining the F1 progeny (RNAi can be heritable).

12 h is the shortest I have analyzed. It surely depends on the gene and I would just check with qRT PCR when you get a sufficient knockdown.