Hitler's chances were entirely slim with the entirety of Operation Sea Lion, had it been initiated. Not with only the Reich's navy and air force anyhow (hence why Britain destroyed the French fleet). Once he opened the war up on two fronts, then he had no chance.
But like Hobgoblin said, he could have had better success with an invasion of the USSR, had he actually been in charge. Although Hitler carried himself with an undeniable swagger, because everything he touched turned to gold. People usually forget, he was actually, a very diabolically clever man. Had he actually been in charge during the invasion on the Eastern Front, he could have seen more success. As pointed out reserves and whatnot. Hell, even managing the largest invasion in history, the Nazis got pretty far, had they began in May as initially intended, there is the very real chance they could have conquered the country. They couldn't have held it for long, a year at the very most, but still, in that time, the USSR would have been decimated and disunited without a totalitarian/autocratic they had been accustomed to for centuries.
Then, he'd have been nigh unstoppable for the United Kingdom. It'd have been a much tougher fight for Britain and America though, that's for sure.