Well it depends on what kind of game you want to play.
So here is the deal with raising threat levels.
1. If you *only* raise threat on Xibei San Ma, and you then invade the Ma's, once you annex them, you'll end up with a neutrality greater than 0, which will mean you can't likely declare war on anybody else - especially not on any other minors. So, it's best to increase threat on Nat Chi regardless of whether or not you intend to fight them first.
2. As far as a date estimation - it depends on when Japan fires its invasion of Nat Chi, and how well your spies can stay alive in Nat Chi (it will be a little challenging). Usually Fall of 1937 or springish 1938 you should be at or very near 0%. Sometimes, you will end up still having neutrality to burn when Japan declares war on Nat Chi - in that case you'll be in for a more minors oriented game because your neutrality won't be low enough to betray nat chi and quickly snatch up some of their VPs lol.
Here are a few additional things to look out for:
The more you declare war the higher your threat will be, if your threat gets too high Japan or the UK may declare war on you. You don't want that until you're ready.
If you join a faction (usually Commintern) every declaration of war from then on will generate x2 the amount of threat it would've before, so you should avoid joining a faction until China is more or less united or you will throw the game completely off the rails and likely bring an DoW from the UK.
If you do go to war against Japan at some point, make sure you're not a Comintern member when you declare war on them (or that Germany is defeated) or you will draw the USSR into a two front war, and the AI, which only has a 50/50 shot of beating Germany in most cases, will almost certainly lose Moscow.
If you take Sinkiang, you can send troops to the USSR to help - the AI almost always rejects expeditionary forces, but if you're a Comintern member, you can just set up an AI theater and give it an objective in Moscow, and earn some free exp on your units and generals - and hopefully help the USSR to not fail.
The guide linked a post above yours says to delete the army and make a new one. Is this really a good idea? I don't have much IC in the beginning, to pump out a new army.