Tongue was definitely in cheek with the "Radio Nav" answer - I thought I'd let the more recent learners give the latest wisdom
I reckon waypoints any closer than 30nm (unless you're turning) are just overegging the pudding. If you pick reasonably recognisable waypoints (avoiding the well-known pitfalls), then you will see them after 30nm unless your course-holding is dire or the wind is horrendous.
Over very flat terrain, I'd go for 30nm (no closer). If there are mountains etc around that give a good "non-waypoint" guidance, I would probably stretch it beyond that.
Flying across Arizona from LA to Grand Canyon, I had waypoints at least 60nm apart and never a moment of "uncertainty". Mind you, that canyon is pretty obvious once you get anywhere near it.
The VOR on the airfield was nice, though, to give that extra warmth.