... How often should I be turning the necks? Once? After every firing? Every other firing? ...
Based on the way you asked the first question, it sounds like this is your first foray into neck turning. If I am wrong about this, please excuse me. this is really long, sorry. To answer you immediate question: you turn once. Dreaded donuts are a separate issue.
When I started to shoot benchrest (this was not recently - we used dinosaurs fat for lube) we started with Lapua 220 brass and used a mandrel to expand the neck to fit .243 bullets. The necks were still pretty thick (about .011) so the loaded round neck diameter was maybe .265 (.243 + .011 + .011). Problem: at this time, 6PPC was a wildcat and the chambers on these guns are especially made with a small neck diameter - the necks were typically .262. So a .243 bullet, plus a neck that is .011 on each side is .265 and you cannot jam it into a .262 chamber. We turned the necks to about .0085 in order to get them to fit. So .243 + .0085 + .0085 = .260 or about .001 clearance all the way around. Not very much. But the small clearance let us shoot the gun and it helps to align the bullet to the centerline of the rifling and we though that helped with accuracy.
Please note, if we didn't turn the necks, we couldn't chamber a round so it not was a choice.
But there are problems. A grain of sand in the chamber neck would require us to force the round into the chamber and screw up that concentricity we were seeking. As the gun warms, the barrel expands and the chamber neck might get tighter. If the brass got long and the neck hit the lede, it would crimp the brass into the bullet and that can get exciting. In matches, we shot about 10 rounds, then reload in the shed and shoot the same 10 pieces of brass. If we didn't get all the crud off the neck - we had no clearance and high pressure and bullets went out of the group. Fortunately BR guns are really strong so these things did not cause issues but the assumptions are not necessarily true for a PRS shooter - especially the part about dirt.
If you don't need to turn necks to make clearance and if you are shooting decent brass that is more or less concentric, try shooting without turning. Here are some reasons:
You spend a bunch of time getting that brass prepped - if you turn necks you cannot put that brass back on. If you test and discover that turning was not helpful, you have to treat that brass special forever more or toss it.
Turning reduces the size of the loaded brass. If the brass fit before you turned it, you just increased the clearance. Think of this like you just had someone ream the neck on your chamber. If a tight neck is supposed to help with accuracy, you just did the opposite.
If you have brass that really needs to be turned - large variance from one side to the other - you might be better off to buying good brass rather than spend a lot of time trying to make a silk purse out of a sow's ear and having to pay for the cutting and measuring tools to do it correctly.
I know how to turn brass. I have the tools and the time. I have some pretty good 300 win mag brass but the necks were pretty bad - roughly .012 to .015. I turned the necks to about .012. They shot a little better. Maybe. But I also have some that has .013 necks and .011 and .010. So every time I shoot, I have to measure all of the necks then use the right button to get the right neck tension. I am experimenting with an expander mandrel to control neck tension but even so, it is a pain in the tooties. I have some better brass and it shoots even if I don't turn the necks and it is a lot easier to hand load. I plan to buy good brass. If I cannot find any, I know what to do but I won't spend the time unless it really need to.
If you decide to turn necks, just do a few pieces - 10 or 20. Number every piece and shoot groups with it before and after. Try to talk yourself out of doing it. If you fail and neck turning looks like a good thing for you, you will have a solid basis to justify the time and cost. if you do talk yourself out of it, you will know that neck turning doesn't help YOU that much with THIS WEAPON and THIS BRASS.