Lots of jokey comments here...
In "Rebel Winter" The vostroyans are allowed to return to Vostroya after 10 years of service. An old grizzled sergeant was explaining to a new recruit that everyone says they are out in 10 years. but after that much fighting with your brothers in arms, few of the ones that make it that long choose to go home.
This would indicate that quite a few Vostroyans (which I will point out are not PDF... not "hard" guard) live 10 or more years.
What really matters here is what regiment you get put with. How skilled your regimental commanders are, and what battle groups your regiment gets assigned to, and how skilled the battle groups high command is.
What type of regiment you are assigned to also plays a role.
I would say that a guardsman posted to an infantry regiment in an ultimately successful theater of war has a good chance of living through the campaign. Better than 50/50. That number would go way up for the member of an armored regiment and would go up even higher for a member of an artillery company. Grenadiers and members of Storm Trooper companies probably have the worst survival rate outside of conscript regiments.
When guard start to lose campaigns, their survivability drops into the toilet. Unlike modern western military, it doesn't seem like the glorious imperial guard makes plans for egress. They tend to make logistical calculations prior to action, then just prosecute that plan blindly. It seems like vox communication is particularly poor universally in the fluff. The chain of command is frozen rigid, and interpretation of orders and initiative is definitely frowned on.
I think the average guardsman's career goes successful campaign, successful campaign, successful campaign, dead... You might get a lucky streak of wins, and live a long long time. Or your first action might be a poorly planned poorly executed advance. Cut off and obliterated..
As an officer, it would probably be just as deadly all the way through company command. I don't think your bacon gets saved at all until you make it onto the regimental commanders staff. Even the regimental commander and his staff would be expected to be in the midst of the battle if they were ever involved in a full regiment-sized action. Which i bet would be pretty rare.
Once you get plucked out of your regimental command into the Departmento Munitorum, the only thing that is going to stop you from living 500 years is a commissar's bullet.
I think its best to summarize a guardsman's life expectancy as... Winners live, losers die...