He didn't count on France falling in six weeks. And he also should have known that Hitler had no desire to occupy Britain, but merely to neutralize it, which he had pretty much achieved.
And Hess flew to England in his Me-110 just a month before Operation Barbarossa to negotiate a separate peace with British nobles. Of course his mission was misguided speculation and no British nobility would have ever agreed to such terms.
That is why he was kept in Spandau Prison alone and incommunicado until he died in 1987. One completely monitored and censored letter per year to his wife. No war discussion allowed.
And it is just a -coincidence- that the one time the Russians voted for his release in later years, the British government vetoed it. The Brits didn’t want him talking about the invitations and welcomes he had.