When the seventy-year-old at the back of the large auditorium with the cheap, ancient projector can read them. Alternatively, when your boss stops complaining. Lines are too thick if they overlap; dots are too big when you can't easily tell the difference between high and medium density. (And if this happens at the default dot size, switch to a colour scale.)

If you're doing PowerPoint or similar presentation tools, you want your axis labels to be the same size as your bullet-point text. One trick I sometimes use is to whiteout the axis labels in the image file of my plot, and put them back in using the same text tool that's creating my bullets.