Yves Dupertuis and colleagues at the Geneva University Hospital looked into this.
They found that providing the bags are multilayered and that all the air is removed from inside the bag. Meaning there is no oxygen inside the bag when the leaves are stored, vitamin C can last up to about 68 hours.
Brief summary of the results:
Vitamin C degrades very quickly, within minutes, when stored in ordinary EVA (ethyl vinyl acetate) bags. Vitamin C is almost completely degraded after 7 hours in EVA bags.
The same applies if there is any oxygen left in the storage bag.
Vitamin C can last for up to about 68 hours in multi-layered (ML) plastic bags stored at 4 degrees, and about 24 hours stored in ML bags at 21 degrees.
I don't know what type of bags supermarkets use or how long the leaves are in the bags before they go on the shelves. My guess is about 110 hours from field to plate.
As far as I can tell - it's not easy to find accurate information on this subject. Most salad leaves are about 4 days old when they get to the supermarket shelf. So there's a good chance that all the vitamin C will have gone from the leaves.
It does seem that vitamin C degrades very quickly. Once the bag is opened I would guess all the vitamin C will be gone within an hour or two.
On a positive note they found that other vitamins are more stable so these leaves are still good for you.
I don't know if you can find any more information about this?