These are Hellblazer's current stats:

Level: 456 Health: 1132 / 1140 Strength: 1133 Defense: 1138 Speed: 1137 Intelligence: 622 Tier: 8

Is it too soon to be focusing purely on health from this point on? Or should I just continue with an even build until Tier 10 or so? Also, when did most people give up autotraining and just do it manually? I'm down to 5 sessions a day with autotraining, and that's starting to look like less and less bang for my 25 CSC. :P

nDepending on where youre aiming with stats, I'd start around now. I started with my pet at 1k even stats, roughly.

You can start focusing more on Health, if you would like, but you shouldn't devote your focus to it entirely. I would say that it's particularly useful to start delegating your efforts towards particular stats around Tier 10, even more so at Tier 11, and a matter of necessity at Tier 12. (Well, unless you want your pet's final build to be even but personally I don't think an even build is the best build for those top-end challengers... Which build is the best probably depends, at least somewhat, on your battling style. But I am not an expert, or even a proper novice, in this area.)

As for auto-training, I wouldn't be using the option any more after my pets got down to 7 sessions, but that's mostly because I am pretty frugal with my CSCs. I would say that auto-training loses almost all of it's advantage when the pet is down to 3 or 4 sessions a day. (That's when boosting and EXP become major players.) It's debatable at 5, but if you can afford it then go for it. It will still some (small) amount of distinguishable difference in the speed of your stat gains as compared to training by hand.