When the rape was reported soon enough for evidence to be collectable; and when it was a form of rape that leaves a distinctive mark relative to consensual sex. I do not know how prototypical these both of these conditions are; but at most, you would reduce the "usual" qualified to "non-negligible", as there certainly are cases that do not meat the criterion to have a usable rape kit.

(Unrelated note, in writing this comment, I tried looking for relevant statistics; but found I have no idea what to even search for. Anyone have anything?)