Realized I screwed up the last submission.
I haven't been able to find much written on the Eastern air war. From what I understand it wasn't really a strategic battle and was more about local air superiority. That said, when and why did the Russians get superiority over the Germans?
From what I've read, I would say around the end of the Battle of Stalingrad.
By the time the German forces had reached Stalingrad they were already overstretched and the Luftwaffe although still having air superiority suffered numerous losses numbering around half the bomber force and about 40% of their fighter force. I would posit that because part of the Luftwaffe force committed to Stalingrad had to be diverted Africa and the losses sustained; by November during Operation Uranus the Luftewaffe had lost air superiority.
Although qualitatively the Luftwaffe most likely had better pilots, the German air power in the Eastern front had diminished massively by November 1942 with the diversion of German air power as well as the losses sustained in the attempt to capture Stalingrad.
Given the reported figures during Operation Uranus, the Luftwaffe Airpower numbered at around 760 or so planes. The VVS numbered around 1500, almost double the aircraft. It's arguable that the Russian pilots were not as well trained, but numerically the superiority of the Luftwaffe had faded by then.
Sources:
The Russians started out with obsolete aircraft and not enough of them along with poor training, poor morale, and poor air-to-air doctrine. Their leadership was very poor due to the purges. Things improved as the US and Britain provided aircraft and technical assistance to the Russians over the course of the war. Russian armaments, engines, and airframes improved, as did doctrine. Pilots got necessary combat experience and leadership improved. Aircraft were produced or donated in large enough numbers to replace combat losses.
The Germans did not have that luxury. Their aircraft improved over the course of the war, but for the most parts the airframes had not improved significantly. The designs they used in 1943-'45 were largely the same as those used in 1939 and 1940. Further, they lost a lot of their best pilots to attrition and the replacements were so inadequately trained that their combat losses were matched by losses to accidents.
I'll recommend a book on the subject. I found it to be very informative, and the VVS and Luftwaffe are included among others. http://www.amazon.com/Why-Air-Forces-Fail-Anatomy/dp/0813123747
Edit: odd word in an odd place.