At what distance wouldn't the Sun cast a visible shadow anymore? How far from the Sun would a (dwarf) planet have to be so that the Sun is dim enough that it doesn't cast a shadow when you're standing on the planet? What magnitude is the border in brightness beyond which a shadow isn't visible anymore?
This depends on eye biology, so there's no purely astronomical answer.  I'll note that Venus is just about capable of casting a shadow (at greatest elongation, with ideal conditions) so let's say that a magnitude -5 object is at about the biological limit of casting a shadow.
The sun has a magnitude -27, so that is 22 magnitudes brighter than Venus.  5 magnitudes is a factor of 100 in brightness, so 20 magnitudes is a factor of 100000000 (100 million)  and 22 magnitudes is about 600 million.
But light follows an inverse square law, The square root of 600 million is about 25000.  So at about 25000 times the distance of Earth, or 0.4 light-years, the sun would be as dim as Venus at its brightest and perhaps marginally able to cast a shadow for someone with acute vision, on a good matt white surface. This puts our astronaut in the Oort cloud.
There's a good deal of uncertainty in this estimate, based on eye acuity. So let's say &quot;something between 0.1 and 1 light-years&quot;