If I am correct it is level 5, so it goes slow, but it goes. Got 1 other egg though first had to train my caterpie to a butterfree Edit: Level 6 with this post XD
Eggs evolve at level 40, they require 5 posts per level, and they start at level 1. So, (40 - 1) * 5 = 39*5 = 195 posts are required to hatch an egg (easily double the amount required to hatch most basic species, save ones that evolve particularly late).
Mystery Egg is going to be lv1 for a while lol, but my second legendary egg is now lv17! Almost halfway there.