Q:

What is Superman's "normal" lifespan under the effects of a yellow sun?

Assuming Superman doesn't die in battle, and lives out his life under the effects of our yellow sun, what is his expected lifespan? Has DC Comics ever provided or suggested an answer?

A:

There is definitely no consistency in how long he is expected to live. 
As Dharini said in DC One Million he is alive: 

In the DC One Million series (1998), Superman's Fortress of Solitude in the 853rd Century resides within a tesseract located at the center of Earth's sun. By this time, Superman has lived in self-imposed exile within the Fortress for over 15,000 years.

In the Kingdom Come Epilogue he seems destined to live for a little more than 1000 years.

In the epilogue, Superman is shown to have survived 1000 years after his return, and as an old man, he watches the fly-pass of the future generation of superheroes.

In the Batman Beyond episode "The Call" the 50 years in the future Superman is shown to have grey highlights in his hair, suggesting that he may only live to be 200 or so.

Bruce Wayne: I could use some Kryptonian DNA.
  Superman: You'll outlive all of us Bruce, you're too stubborn to die. 

In the TV series Smallville in the third season a highschool kid who has the power to see people's deaths touches Clark Kent and sees him just traveling on forever. The implication is that in the Smallville universe Superman will never die. 
Paperjam makes the point that Superman may die as soon as the Sun transforms into a Red Giant. Superman could ultimately leave to a different yellow star if his intention is really to live forever. 

A:

I can't find any canon sources which states his actual lifespan under the yellow sun, but in the 1998 comic story "DC One Million", Superman was shown to be alive in the year 85,271. So he could live at least that long. 
But he is hale and healthy then, so it might not be the maximum.

