How far would you walk to fill out a Pokedex entry?

I wouldn't bother. You'll get Porygon2 eventually and like you say it isn't much use. Appreciate that Blissey! I don't feel like I'll ever get one (2 Chansey caught, none hatched)

Not walking with Chansey is too high of a cost. Not worth XS

You will eventually hatch one.

If you walk it, then the hatch will be a disappointment, and make you feel like you have wasted 'buddy distance'.
If you don't walk it, then the hatch will still be welcome, and even a highlight.

Like everyone says, not worth to walk a mon just to complete the dex, I learned it during my walking sessions on gen1 to get mon like Machamp, Golem or Charizard just to see them become a nest, hatch, or events.

There is no way you can complete the hole dex without travelling to a lot of countries (at this point probably very few people in the world have all region exclusives from gen1 and gen2), so what if you have porygon2 missing space for a few more months? Nothing will change. Walk the most valuable mon always, the ones you will use either for offense or defense, Porygon is a rare mon for a lot of us, yes, but not worth the effort for me.

I HATE walking with a buddy for more than 100 kilometers, however, except for Magikarp.

Wait for Gen 4, you will need only 124+1 candies to evolve it to PorygonZ.

Porygon2 will cost 25 candies to be (gen 4) evolved, just like (gen 2) Golbat.

Like 333-blue said, it's not worth it, just wait. Before Gen 4 is released, Niantic WILL do an event where there will be naturally spawning Porygon2! Mark my words. Just like they did with Chansey before Gen 2. Walking for a not so useful buddy is just a waste. You are better off walking Dratini, Larvitar, Chansey, Snorlax, Lapras and their evolutions.

0KM. I would never walk a pokemon that isn't a strong defender or at worst a strong attacker.