Cellular demand for antioxidants
Most experiments to try to get at this type of question use stable isotope tracers to measure NADPH production from the various pathways that can make it, and then subtract away the amount that's predicted to be needed for growth/anabolism (based on the observed growth rate and biomass composition). I think this paper is by far the best example:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4104482/. 

Quote:


  The overall demand for NADPH for biosynthesis is > 80% of total cytosolic NADPH production (Figure 4b), with a majority of this NADPH consumed by fatty acid synthesis. At least in transformed cells growing under aerobic conditions, most cytosolic NADPH is devoted to biosynthesis, not redox defense.


Note that these are fast growing cells without additional oxidative stress. Interestingly, what the authors find is that imposing additional oxidative stress doesn't actually change NADPH production that much, but it slows down growth as more NADPH is used for anti-oxidant defense (Extended figure 9).

For skin cells, the situation might be a bit different, at least in terms of the increase in NADPH production upon oxidative stress. You might look at this paper: http://www.cell.com/molecular-cell/fulltext/S1097-2765(15)00456-6