I've been searching this forum and elsewhere, hoping to find a particular little nugget of info, but without any luck. So my question...

At what distance or magnification one should switch from a "normal" lens to a lens designed for more close-up work? How close will most "normal" lenses focus and still render an acceptably sharp subject? (I know this will vary a bit from one lens design to another, but is there any general "rule of thumb?") As usual, thanks muchly!

(And let's see if I've gotten my name back to normal after my high school students switched it...)