The most I have ever saved up was 5 - 10km at a time. And it took a long time to collect those 5. I usually have about 4-5 eggs incubating at a time. And that is buying incubators with gym money. 100 coins a day.
I would rather just keep hatching eggs and use the dust/candies/mons right away than try to save up 10km eggs for a big hatch. I don't use lucky eggs on them so no point saving them.
If I get a 10k egg I put it into an incubator right away in hopes of getting a good Dratini. Larvitar, Snorlax or Chansey. Can use the candies for sure.