system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40K] How much fighting does the imperial guard do against other humans compared to the amount of fighting they do against xenos/chaos threats?

What I really want to know is how likely is a solider to see combat against what?
gpt
The Imperial forces are more likely to see action against Rebels, Separatists and Heritics than any other threat. These are ongoing struggles throughout the Imperium, even though they somehow manage to keep knowledge of Chaos on the down-low.

The next most common threat are the Orks. They are everywhere, and almost impossible to exterminate. The majority of these encounters are likely dealt with by PDF formations as they are little more than feral Orks who haven't reached the population necessary for Meks to manifest. 

Next come Tyranids. These are increasingly common on the Southern edge of the Galaxy, though wandering splinter fleets have been seen as far north as the Eye of Terror.

The least common of the major powers are the Eldar (both types, though Dark Eldar are more likely to attack human worlds) the Tau and the Necrons. These powers, while mighty, are either too localised or too disinterested in the Imperium and it's worlds. Conflict only comes when the Imperium is the aggressor, or under rather specific circumstances (neutralizing threats, trespassing, or even helping against common enemies).