On average how fresh are chicken eggs before they are delivered to the
store for sale to the public.

I will answer this assuming you are speaking of America as most users
are from there?

From AEB.ORG they say 'most' arrive in stores withing a 'few' days ...
personally I would take that to mean less than 7 days old...

Freshness 
How recently an egg was laid has a bearing on its freshness but is
only one of many factors. The temperature at which it is held, the
humidity and the handling all play their part. These variables are so
important that an egg one week old, held under ideal conditions, can
be fresher than an egg left at room temperature for one day. The ideal
conditions are temperatures that don't go above 40°F. (4°C.) and a
relative humidity of 70 to 80%.

Proper handling means prompt gathering, washing and oiling of the eggs
within a few hours after laying. Most commercially produced eggs reach
supermarkets within a few days of leaving the laying house. If the
market and the buyer handle them properly, they will still be fresh
when they reach the table.

It is not true that freshness can be judged by placing an egg in salt
water. A carefully controlled brine test is sometimes used to judge
shell thickness of eggs for hatching purposes but has no application
to freshness of table eggs.

How important is "freshness"? As an egg ages, the white becomes
thinner and the yolk becomes flatter. These changes do not have any
great effect on the nutritional quality of the egg or its functional
cooking properties in recipes. Appearance may be affected, though.
When poached or fried, the fresher the egg, the more it will hold its
shape rather than spread out in the pan. On the other hand, if you
hard cook eggs that are at least a week old, you'll find them easier
to peel after cooking and cooling than fresher eggs.

In addition...

Here in Canada, eggs arrive in stores 'between 4 and 7 days from being laid'...

http://www.eggs.ca/eggfacts/eggstore.asp

In Canada eggs have a 'best before date' stamped on the box which is
approx 35 days from the grading date.  So when you are at a store you
can base the freshness of the eggs on that date.

Some suppliers are now printing a 'lay date' directly on the egg using
vegetable inks.

I can tell you from personal experience that a freshly(same morning)
laid egg from a free roaming chicken tastes VASTLY different from the
eggs you buy in supermarkets. It has a much gamier taste. Much
stronger and really a whole different animal (scuse the pun)
Personally I prefer the taste of supermarket eggs  but my mother
prefers freshly laid eggs.