The attacks in 1940, 41 and 42 had little effect because they were on such a small scale. Bomber Command dropped 13,000 tons in 1940, 32,000 in 1941, 46,000 in 1942. To put those figures in perspective, the Luftwaffe dropped about 50,000 tons on the UK in 9 months of attacks between the start of the Battle of Britain and end of the Blitz.I see little correlation between the bombing attacks in 1941, '42, and '43 with the German surrender in 1945. It wasn't until the destruction of the Luftwaffe, the attacks on the oil industry, and maybe the transportation system, all in 1944, that I see bombing attacks and results that are making a difference in how long Germany can keep fighting.
It wasn't until 1943 that the attacks became large enough to have a significant effect. In 1943 Bomber Command dropped 157,000 tons and in 1944 526,000 tons. German armaments production, which had been growing rapidly, stagnated from the summer of 1943 until early 1944, when Bomber Command were diverted to support invasion preparations.
The mid point for bombs dropped on Germany was September 1944. Half the bombs were dropped in the 5 years before September 1944, half in the 7 months after.