How long would it take for the WAllies to achieve air superiority to even be able to successfully use nuclear weapons against the Reich after it's defeated the USSR?
Air superiority for? The Allies flew thousands of reconissance missions over Some were sucessfully intercepted, most werent. If the air forces run a series of photo recon missions with the same profile as possible atomic missions then odds are the Germans will ignore most them the same as the Japanese did. Three radar blips at high altitude on a routine flight pattern are a low priority.
OTL The Allies gained air superiority east of the Rhine almost as soon as enough P51 groups were available. It was a matter of range & the P52 with drop tanks allowed the 9th AF to defeat the main German interceptor force over Germany. In October 1943 the ball bearing raids that were unescourted into Germany took severe losses. Three months later in the Big Week of January 1944 it was the German defense that took the severe losses. Changes in escort tactics, improved bomber mission profiles, improving veteran bomber crews all helped, but once the number of escorts hit critical mass it was game over.
A mass transfer of aircraft from the east is not going to help. Part is because there was no 'mass' to transfer, and part because this would accellerate the German aircraft losses. From 1942 thru to late 1944 the German air force was able to maintain a average of a little over five thousand operational aircraft on all fronts combined. 'Brute Force' by John Ellis has biannual charts showing the numbers. At some points the number hit highs above 5,500 combat worthy aircraft. Other times it fell momentarily towards 4,500. At late 1942 approx 60% of this operating force was on the eastern front. As the 1943 campaigns developed those in the east were reduced & in the west increased. The most significant change in raw numbers was when the eastern front was stripped of interceptors to defend Germany. Well over half the flyable interceptors were shifted west. By mid 1944 the proportion of operational aircraft in the east had fallen below 40% of the total & the proportion of interceptors there were far less.
The other half of this is the west Allies were far better or more efficient at destroying German aircraft. For the 12 months of 1943 two thirds of German aircraft losses were in the west. This despite that for most of the year the west had roughly half the total aircraft. The bottom line is that when the interceptor groups were transfered from the east to defend Germany the pilots died faster than vs the Red Air Force. Maybe I can post some of the relevant numbers for this soon.