Currently Bucky's stats are as following:

Level: 83 Health: 212 / 212 Strength: 231 Defense: 206 Speed: 214 Intelligence: 1010 Tier: 3

I was wondering how far do I have to auto-train him till I reach the point when auto-training isn't worth it anymore and train him normally?

And would it be still worth it to start (again) auto-trainining Clint who's stats are as following: Level: 616 Health: 1240 / 1240 Strength: 1252 Defense: 1240 Speed: 1240 Intelligence: 1204 Tier: 8

Sorry for all these questions but I really want to know these things for sure before I train them further.

With major CSC inflation in the past few years, I would say autotraining is really only cost effective until your pet's level is 350 (beginning of T7). After that, boosters would be cheaper to buy.

Thank you for your reply :) I will keep autotrain Bucky till tier 7 and train Clint with boosters.

Don't forget to do your Battle Quests everyday, too :D And Quentin's quests give out mage amulets sometimes.

scratchcards can give stat boosts which come in handy as well