I was just thinking about nightsticks, as long as you're going to allow unlimited stacking, you might as well do it right. Make a custom nightstick, and put the extra turning ability on it twice, the second instance should cost 3/4 as much as the original. Now you've got an 8 extra turns per day nightstick for 1.75x cost. Now presumably its giving the extra turning feats 1/day, so multiply the cost by 5 and it provides unlimited uses of +8 turns per day.
Now you can persist 1000000000 spells for something like 66k gold. Fair and balanced, right? Its just as reasonable as infinite stacking nightsticks, without the retarded concept of a guy sticking a bunch of rods in his portable holes.
What's more, you can share this one rod of ultimate cheese with the rest of your cheese brigade, since you only have to possess the rod while buffing, so now you can split the cost with the cheater of mystra and twice betrayer and whoever else is rollin on the tier 1 cheesewheel with you.
Even if you don't go with the x5 for unlimited uses, you might as well get the 50% discount for stacking the same effect on a slotless item, and there's no raw problem with sharing the rod with your cohort and group either.
edit: wow, this argument is old, like 2-3 years old. best gem I found is this, if nightsticks stack infinitely, so do +1 caster level ioun stones