"gbaikie was wrong about the ~ billion years. Even the slowest accretion scenarios for the terrestrial planets take just a 100 million. Other scenarios are quite quick - for example the Capture/Collision scenario has the initial giant planets collapsing and forming moons within ~ 1000s of years. The two innermost planets collide and their cores form Earth/Venus - Mercury, Mars and possibly Theia (the Moon-maker), were satellites of one of the planets. Mercury was close enough to the collision to be abraded down to almost its core. Mars was battered enough to have its current hemispherical anomaly. All this occurred while a captured disk decayed away around the Sun within ~ 3 million years."

Here is a reference:
http://scienceweek.com/2004/sc040903-1.htm
It does look like current thinking is the Sun is much younger than 5 1/2 billion and it's about 5 billion or less:
"1) The age of the Solar System as a whole is easier to determine than the age of Earth. The age of the Solar System is reliably inferred from the age of *refractory element-rich inclusions in meteorites to be approximately 4.57 billion years, thus providing an upper limit to the age of Earth. " [from above link]. I didn't intend to imply that earth [or proto earth required a billion year to form- but rather I was indicating there was a long period in which this could take place [the proto Earth forming and it's meeting the proto Moon. BUT I was wrong according to the current theories and seem to indicate that there had to be a very short time period in which this could have occurred.

Now I do have problem with your assertion that "Even the slowest accretion scenarios for the terrestrial planets take just a 100 million."
Now I would think the grand bombardment period which often said to be 1/2 billion year period of time could considered part of accretion time period of Earth. Now perhaps you have some information that shorten this bombardment period to less than 100 million years or you can explain why lots of space rocks crashing down aren't considered accretion.

Now, I do understand that much stratching of head & worry is done in the area of getting bodies up to the point of having a significant gravitation affect [according to the thinking] and so once this stage is achieved one might rightly call most of the accretion process over with.

But even today the planet adds tens of tons of space material each year- so that's the other end of the spectrum.
But I would say that the accretion period would the time when most to the planetary mass is added. And I would say this time in which most of the planetary mass was being added would be the same time as the grand bombardment.

Let put it to you a different way if had the conditions associated with grand bombardment which were occurring today our rate of impactors hitting Earth would be greatly increased. And I would assume that we get a similar pattern as we get today: the smaller stuff are much, much more common than large impactors. So in today's world we get the really big monsters every 100 million year or so; and the little stuff occurring constantly. If we get instead the monsters in a yearly time frame then also we get the little stuff being somewhere around 100 million times the current level, that's billions of tons daily raining down- I would call this a period of accretion- especially when over a period of thousands of years.

In summary if accretion can only be at very most be 100 million years, then the bombardment period would be less than 100 million years.

Or as some wizard said: "They are one, the ring and the dark lord."