At what trainer lvl should I start evolving Pokemon?
Obviously as you increase your trainer lvl you encounter higher CP pokemon, so it doesn't make sense to power up or evolve something like your CP 20 Charmander. At the same time, leveling up becomes exponentially more tedious as you reach level 20 and beyond, hence the CP of pokemon you encounter in the wild doesn't increase as quickly anymore. Considering this, at which trainer lvl would it make sense to start training your pokemon instead of waiting to find a slightly more powerful one in the wild?
Obviously as you increase your trainer lvl you encounter higher CP pokemon, so it doesn't make sense to power up or evolve something like your CP 20 Charmander. At the same time, leveling up becomes exponentially more tedious as you reach level 20 and beyond, hence the CP of pokemon you encounter in the wild doesn't increase as quickly anymore. Considering this, at which trainer lvl would it make sense to start training your pokemon instead of waiting to find a slightly more powerful one in the wild?
Answers
Lucky egg evolution spam is the most efficient way to level up. That being said, the most efficient way to level up also entails never evolving to a third stage.
Your losses are partially made up for by the pokedex XP, but generally speaking, you'd only want to evolve to stage three forms around level 20 or not at all. The only reason you'd need strong pokemon is for gyms after all, so outside that, they're mostly just for your satisfaction.
There's always Eeveelutions (among others) anyway, who still result from optimal evolution spam and get quite strong.
Ryik is right that you should want to evolve those of your pokemon that you want to in an en masse evolution spam, and that specifically choosing to evolve say, a lot of Weedle into Kakuna instead of a mix of Weedle into Kakuna and Kakuna into Beedrill is very effective for purposes of XP gain.
That said, the pokemon you obtain at a low trainer level and so possessing a low level and cp are not at all useless, because they could have more end potential than pokemon of the same species or evolved from it that you encounter later. A pokemon's IV's and moves speak to its relative end potential within its species, its initial level at time of encounter does not.
Evolving and leveling a pokemon can be done at any time: the truly optimal times are when you have to because of a gym in your way, or when you're evolving everything with a lucky egg. Evolution in this game is the same as evolution in the main games: you're not changing any of a pokemon's total xp, its level, or what it requires to get to the next level, you're only changing its base stats.
The only rule is that if a pokemon evolves, it's probably better to evolve it before leveling it up, as you don't know what moves the evolution could leave it with.
