I think a pint s good for anything :) Chin Chin!

How long was the BoB to last? Well...... too bloody long for Hitler so he cancelled the gig!

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!

The moment they (Germany) went defensive the Battle of Britain was over.
Speaking of battle, it is highly likely the individual pilots would have a sense of an endless struggle. The only comparison they had was WW1 and the channel was one big nomansland, so it is easy to imagine some pilots belief that it would last for ten years as conjected by the OP.
But i seriously doubt it was a universally held opinion. ;)