After verifying all my scanned negatives and slides I've found that at 3200 dpi grain is visible in Ektachomes from the 60`s and early 70's, Agfachromes up to the 80's (this were really grainy) and even some Fujichromes from the early 80's (in this case the grain is compact and uniform and of little importance) In all these cases the grain was also visible under a microscope at 30 magnification. In modern slides (I use Fuji's Sensia and Provia) even at 4800 dpi no grain is visible.
With colour negatives my results are similar, old negatives are notoriously more grainy than modern ones. With some negatives from the early 80's grain is plainly visible at 3200dpi