When is the right time to start use StarDust and training

Like Samthere said, you can delay indefinitely, however id recommend keeping one Pokemon high to take gyms with. Don't power it up more than you have to but keep it high enough to get at least a few gyms. The extra star dust and coins you get should be worth the candies and stardust you spend to keep just one Pokemon high. I usually get rid of anything with a CP meter under 90%. It's probably good to set a goal like level 25, otherwise you probably will delay it indefinitely. Lastly, go ahead and evolve any Ratatas and Spearows (or other really common ones in your area) you can for the experience. Their evolved forms max out pretty low so they aren't really viable later, otherwise leveling will get really slow unless you are hatching eggs left and right.

Personally, if you are starting out playing pokemon go, don't even use stardust until your level 15 or higher.. preferably 20. I can't even begin to explain how much candies and stardust I wasted raising the CP of one pokemon, just to find one in the wild CP300+ higher than the one I was boosting. I found that very frustrating, so I began hoarding stardust. I try to return from my adventures with at least 7 to 10,000 stardust. But another tip of advice for low level players is to save up to at least 50,000 stardust before using it.

While it is definitely advisable to hold off on powering up your Pokemon early on, when to decide to use stardust is highly situational. There are several things to consider:

• Does your Pokemon have high or perfect internal values (IVs) and desirable attacks?
• Are you a high enough level where the Pokemon you are catching or hatching now are not keeping up?
• Can you compete for gyms with the Pokemon you have?

If I were to ever acquire a desirable Pokemon with a desirable move set and perfect IVs, I'll eventually power it up as resources allow no matter what CP it starts off with.

Speaking for my own experience, I'm level 25 and I've started powering up the Pokemon I use to take and defend gyms... which are mainly Vaporeons (I have a small army of them). Most of them have high IVs and desirable moves (either Hydropump or Aqua Tail). I've stated to fall behind in level a little bit compared to opposing trainers, probably because I've yet to spend a single cent on the game, so all of my lucky eggs, lures, and incenses are from leveling up. So, I've been maxing out some nearly perfect Vaporeons to keep up with the level 28-30 trainers I sometimes see controlling gyms.

But keep in mind, this is simply for the situation I find myself in. If you're still finding Pokemon with high CP and you're keeping pace with the other players around you, there's probably no real reason to spend stardust quite yet. And if you're still at a low level, then you probably want to prioritize gaining trainer experience over increasing your Pokemon's strength so that you can continue to accumulate stardust for when you catch up and want to be competitive at your local gyms.

It's best that you use your stardust at level 31 or 32 because by that time you will almost have 700k of stardust and enough rare pokemon candies. I am level 29 now and I have over 510k of stardust.

I waited until about level 27 or 28 to start upgrading. In the meantime, I kept track of who my highest level and % pokemon were, using IV stats app. I play a game of survival of the fittest with my pokemon, and don't usually keep more than 2 of each kind (unless it's a very desireable pokemon)

Pokemon eggs max out at level 20, so it doesn't really make a difference when you start upgrading a hatched pokemon, if you're past level 20. If it's a good pokemon, you might as well start upgrading. I hatched a level 20 charmander, 100% IV stats. I've been sitting on my candies until I reach 100 to get a Charizard, but I'm getting impatient and started sprinkling stardust on this (now Charmeleon), currently at level 26.

For the time being, I will continue sitting on the rest of my stardust until I acquire a pokemon with at least 93% IV. That's a personal choice, however.

Well at level 30 the CP level of pokemon that appear in the wild is capped so delaying until then will give you the best use of stardust. Pokemon that hatch from eggs reach their max potential at level 20. So I'm saving up my 10k eggs until I reach that goal.

Pokemon CP stops getting higher at level 40 so between 30 and 40 you just have to spend candies....but getting to that level will likely take years with how fast the XP needed rises