how much is enough to retire

Is anyone out there good with figures? My wife and I were discusing, at dinner, how much we need to have a comfortable retirement.

I am 42 and my wife is 39. According to the models around the traps I will have 750k and my wife 1.8 million at 60 years. My wife has a generous scheme. Is this enough? Or in 20 years what amount will need to have an income of around 70k (or in future value terms). My wife seems to think we need 2 million each! Whereas, the models say I am on track.