Consider bolt action, near max loads, full length sizing, 'average' commercial brass.

1. On average, how many times should a case be reloaded?
2. How to determine when a case can't or shouldn't be reloaded again?
3. How does neck sizing only affect case life?
4. What brand of brass lasts the longest?

If you size it to just fit your chamber, it should last the longest. Fire middle of the road propellent charges, anything run at the ragged edge will wear out faster, if you want more speed, get a bigger case. And neck sizing helps brass life, because you aren't expanding ( firing fills/seals the chamber ) and "shrinking" ( full length sizing puts the brass back to a smaller dimension) the body of the brass all the time.

And the brand of rifle please. I have a great article at home, on reloading the .308... the author reloaded ALL the available 165-168gr bullets, with all the available brass, all the available primers...etc.... The overall differences in all the variables ( components) was startling. I'll look it up tonight.

I failed to mention - Rem 700 SPS.  

When do you know its time to throw the case away?

Split necks, signs of case head separation, loose primer pockets.

308 brass will last a loooong time in my experience.  Even at max loads and FL sizing.

I FL size for my savage 10FP and my friend does for his 700 as well...

I think he has brass that is probably on its 10th + reload... mines appraoching that too and its stil running strong, accurate and I haven't thrown away any cases yet... and I load max + loads in it.

I us hornady match brass... hes been using Win and remington.

MIke.

Brass will last even longer if it's properly annealed.

I didn't disagree with anything said in any of the above posts.  There's nothing really definitive there, but good reasonable advice.  One of the best short reads on .308 is from 6mmBr.com.  They answer many .308 questions very well.  Especially note their advice on Sizer dies.  As for the brass that lasts the longest, many swear by Lapua.  I Like Winchester because its almost as good and costs 40% less.  Federal brass while good, is softer and results in loose primer pockets sooner.

Case life is related to brand, Federal SUCKS probably the shortest case life, Machine gun fired mil spec sucks due being fired in sloppy chambers etc, and how you size the brass and care for it is another thing.

For most purpsoses without investing bushing dies, carbide expander balls, and head space measuring tools to aid in setting up FL sizer, and annealing....
I would say 5 to six reloads.  But, if you learn to treat your brass right well you can extend the case life.  I have 19 reloads on 1 lot of 308 Win Lapua brass.
It looks great, primer pockets are snug.