How long would you keep defensive ammo in storage before you would feel like you would have to shoot the old stuff and get some new ammo for the shelf?

It depends more on the storage conditions than the age. High humidity and temperature are the enemies of ammunition and reloading components. I would probably replace properly stored defensive carry ammunition after ten years or so, but that figure is purely arbitrary. The military routinely uses ammunition older than that.

If properly stored, I wouldn't replace it. Firing an occasional test shot after a decade or so is as far as I would go.

I was shooting WW2 surplus 45acp stuff quite recently, it worked fine despite being over 60 years old. A friend of mine has shot some 303 British from WW1 and said that it was fine. I have no idea where or how it was stored. The proverbial 'cool dry place' is a good rule-of-thumb.

Properly stored ammo does not go bad.
I might "upgrade" to a more modern bullet design, but not because of age issues. I have 20 year old ammo that still "stands guard" and have no worries. I have recently fired WWII vintage .30-06 and 9mm, again with no issues.

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Formerly know as Lucky Derby

Camprunner,
I have fired WWI and older ammunition within the last 5 years: 380, 30-06, 45, 38, and 32 all did what they were supposed to do, the 38 and 32 were rim fire blackpowder rounds atleast 100 years old. It would seem that age was not a problem.

I store my ammo in good quality 50 cal military surplus cans. The cans are kept inside (where temp is regulated). Properly stored, quality ammo will go bang 50 years later. A friend of mine bought some WWII era surplus 45 ACP (boxes stamped with 1943 and 1944 dates) in 2000 to shoot in his new Para Ord 45. We shot a lot of boxes. All of the ammo fired.

Keep mine in cans with the temp regulated. I have had no problems.

I bought a bunch of silica gel, (you know, the stuff that says "do not eat" that you get in your electronics) and store my ammo in a sealed can with the silica gel to absorb moisture.

i am told it should last forever. Then again, I live in california and there is no humidity.