system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k]What's the life expectancy of your average...

...civilian?

...politician?

...guardsman?

...Astartes?

...inquisitor?

...psyker?

...techpriest?

etc.
gpt
Civilian? Depends on the world. Most Imperial worlds maintain perfectly adequate standards of living for their citizens. Some are Death Worlds. Comparing Fenris to Terra to Macragge will give you wildly different answers. 

Politician? Impossible to know, same as civilian. A wealthy politicians? Centuries, rejuvenat treatments can extends lives for around 300+ years. 

Guardsmen in combat? 15 hours. (edit: in case this wasn't clear, this was a bit tongue-in-cheek. IG life expectancy will range from hours to decades, it all depends on the conditions of deployment.) 

Astartes? Several centuries. Space Marines like Chapter Master Dante of the Blood Angels that are over 1,000 years old are exceedingly rare, so we have to assume the average age of a SM is something around a few centuries, give or take a few decades. 

Inquisitor? Likely same as a wealthy politician or noble family, they can...acquire augmetics and rejuvenat treatments.

Psyker? That can range from a few hours to a few days to a long and happy normal life. A member of the Adeptus Astra Telepathica, for example, will suffer from early physical degradation and decline, as the toll of using their powers will weaken and age them. A Navigator may last for decades or centuries, until they are little more than cocooned mutants, entombed inside their vessels and preserved by nutrient baths and arcane science. Other psykers will lead normal lives (as much as they can), until something in their mind snaps, a Daemon possesses them, and their entire planet is consumed in the birth of a million, billion gibbering daemonspawn and Chaos corruption reigns supreme. Others will be taken before the Golden Throne itself, bound to the God-Emperor's soul in the infamous Soul Binding ritual, and proceed to spend the rest of their mortal lives as astropaths for the Imperium.

Techpriests can range from standard human ages to thousands of years old. Belisarius Cawl, an Archmagos and no mere Techpriest, has been alive since the Heresy itself. The Adeptus Mechanicus can prolong the life of its members almost indefinitely, provided they are of a high enough rank to deemed worthy of that level of communion with the Machine-God.