Answer 1

It really depends, because once I walked outside of the day care centre and the old man told me that my Ninetails was already holding an egg.

Answer 2

if the 2 breeding pokemon are getting along(if you ask the old man, he'll tell you)every 256 steps a 50%chance.

Answer 3

Once I was breeding Mudkip on White and the old man said I had an egg. Well I took it from him and as soon as a reached the bottom of those little stairs to the black path (about 5 steps) there was another egg. Yet with the same pokemon in the daycare I rode around on the bike for at least 10 minutes before there was another egg. So, even if they are getting along, there are a lot of different time spans between eggs sometimes.