Simply when you can not afford to buy it.
I had around a dozen Televue Plossl's a few years back ranging from 8mm to 40mm. I now own just 4 proper observating eyepieces and an all rounder zoom for Solar imaging (locating). The eyepieces are 35 Panoptic, 22mm Nagler, 13mm Ethos and 8mm Ethos. I would highly recommend all 4, they are expensive, particularly the Ethos. I am sure there are many other eyepices that would do pretty much the same job, these were all purchased 2nd hand at a price i thought i could justify.
That is probably the answer to the question, if there is one, how much can you justify spending? I certainly would not recommend buying a set personally, pick and choose the best ones to suit your scope and interests.