For me I have a few factors that play into when I evolve a Pokemon with an evolutionary stone like what level it's on and what moves can learn if I evolve it at that time or if I wait a few levels,a good example is with Eevee evolutions I normally evolve them pretty quickly especially with learning their STAB moves..sometimes another example is Skitty I always wait until she learns Faint Attack (that is unless her ability is Normalize) and for Pikachu I was almost always (except once in Ultra Sun) for it to learn Thunderbolt before evolving and there's the elemental monkeys normally they are the middle ground once I actually tried to wait until it learned Crunch but now really it depends on the other moves it has..so for me it basically depends on the Pokemon and when I get the evolutionary stones