How Long Was The Battle of Britain Expected to Last?

[senseispcc]

For planes to fly in WW2 they need good weather. This to support the ground landing and operation of the troops you land in enemy territory. Logical see the story of the 6 June 1944 and thereafter.
The objective for the Germans was to invade at least in part of Great Britain if this was the project of the big boss at all! Which is not certain because he already was looking in direction of the east for his next campaign.
So the duration of the battle of Britain is very vague because German did really never end it officially!