How much shelf life do oxygen absorbers add to food?

It's not clear that oxygen absorbers would help you preserve food at all.

First, the science on the effectiveness of oxygen absorbers is still young, and few national food regulatory agencies have offered any opinion as to whether they work or not (just that they're non-toxic). Most published "studies" are written by people with a direct interest in selling them. Second, the oxygen absorbers you get from internet retail are unlikely to be the same kind that food manufacturers would use; based on a quick perusal of prepper websites, what many of them are selling aren't actually oxygen absorbers at all.

Most importantly, though, is that just putting an oxysorb in your jar of granola isn't sufficient. There are multiple kinds of oxygen absorbers, including low temperature ones, high temp ones, ones that absorb moisture, ones that emit moisture, and even ones that emit alcohol. The most serious independent studies I can find come from military forces, and they make it clear that it's necessary to match the correct oxygen absorber, packaging material, packaging method, and foodstuff.

Making effective use of oxygen absorbers, then, would require you to have an inventory of different kinds, and also invest in industrial-grade packaging equipment. This does not seem cost-effective compared to buying replacement buckwheat flour every 9 months, or just buying a heavy duty countertop vacuum packing machine.