I am not interested in hiking Mt. Whitney again this year, but 12 hours round trip in the snow seems way too ambitious to me. The best I could do was 10 hours including an one hour rest at the summit. This was under perfect conditions, wearing running clothes, two bottles of liquid, and running all the downhill sections. The training for that hike included preperation for the Baldy Peaks 50K. This included 10 hikes to Mt. Baldy and San Gorgonio. I don't personally think the affects of altitude kick in until you get over 11,000 feet, but everybody is different.
I think it is great that you are determined to do this hike, but you might plan on more time for this hike, maybe at least 4 more hours not only for the snow and ice, but for the altitude.
GOOD LUCK AND BE CAREFUL
