How long have you been waiting?' 'Only for a minute or two.' 'How long have they been married?' 'Oh, for a very long time. More than 25 years.' 'How long will the concert last?' 'It should be over ...

A rule of thumb is that the transfer to Mars takes around as long as the human period of gestation, approximately 9 months. But that really is no more than an approximate value; you still have to ...