I would start training as soon as there are loads of idlers sitting around, which is going to be at a different time depending on how quickly things get set up. Just giving them a training weapon and a shield, in squads of two, is infinitely better than procrastinating for when metal armor and weapons are available. Traps will likely be enough for quite a while, until some of the sieges get ridiculous and titans start showing up, but you can always just lock yourself inside during an emergency with drawbridges.
Having variety in your weapon choices is good as well. Axedwarves are good at killing sentients, such as goblins, as they chop limbs off like crazy. Speardwarves are good against large creatures that can't have limbs removed easily, as they puncture organs quickly. Marksdwarves require minimal training and equipment, and are often the best choice if good armor and weapons aren't available. Blunt attacks like those from hammerdwarves are good against armored enemies, though there really aren't any in vanilla where it makes a difference, except maybe early on if you're stuck using really poor quality materials like copper. They are, however, preferable to axedwarves against necro'd undead, as chopping limbs is an insanely bad thing when they all start crawling again. They suck at finishing kills, though, unless they get a lucky headshot.
Training large batches of recruits takes a massively long time if there isn't an appointed legendary champion to teach them, as random recruits will try teaching others things they have no skill at. Sparring is much, MUCH faster, and occurs quite often in small squads of 2-3. Marksdwarves are only really trained by combat and archery ranges, but those are often more of a hassle than they're worth. Training on prisoners is fastest, and prisoners' longevity can be increased hugely if they're trained using wooden spike traps.