Okay, basic conclusions first...  

Please keep in mind the stats really aren't that important and there were plenty of factors that I couldn't look at.  Also, every site and its author are unique, try not to compare yourself to the averages - just because a couple peoples stats are one way, doesn't mean that yours will be too - my sample size here is still entirely too small to really draw any hard conclusions.  That was my disclaimer of sorts...

It seems that in the first 8-16 months a site will grow exponentially pretty well.  All that exponentially means is that growth will be really slow at first and then pick up very quickly.  Graphs always help so check it out... this is the average daily visitors for the first 13 months...
avg
You can see how the increases in traffic are pretty small right at the start, but then get larger and larger differences.  The R2 value tells you how closely the trendline (in black) fits to the data - the closer the value is to 1, the better the equation predicts the growth.  I was hoping to see over 0.95, but the variance in exponential growth start times, along with a set of completely linear data from Marc didn't help.  It's still a pretty good fit though.

I suspect that this has to do with getting your site seen by the search engines, as well as the fact that when your site is small, its much easier to increase (as a percentage) its size.  adding 10 pages to a site that has 10 pages doubles its size(100% increase).  If you add 10 pages to a site with 100 pages, its only a 10% increase in size.  In other words, it takes much less work to double your sites size (and hence its traffic) when you are just starting.    

You can see this pretty easily if you take a look at the % growth chart (please note I didn't include the 1-2 month percentage, going from 7 visitors to 40 is a 500% increase and would have made trends harder to see)
avgp
Try not to look at the specific values here, but try to see the trend - growth % starts out pretty high and gradually decreases and holds pretty well even after the first year or so.  So what does this mean for growth after the first year?  It certainly doesn't stop!  So lets take a look at the average visitors graph for months 13 thru 23
avg
This is really good linear growth, meaning you're gaining about the same number of visitors every month.  While the percentage is not very high here,  picking up an additional  50-100 daily visitors every month is much better than having a 60% growth equal to 20 daily visitors.  

Thats the basics.  I guess the take home message here is to not get frustrated with slow growth at first - the largest increases you see happen somewhere around the 1 year mark - once you hit that bit of exponential growth, if you keep working, your traffic will only increase more and more and more... While I can't really say I've reached my goal of quantifying the "snowball" effect - I can definitely tell you that its very real, and depending on how much  work is put in, usually starts somewhere in the 8-16 month range...

I'm not going to post up here any individuals graphs - but if you  want to see it, contact me through my site or email me -  ta [at] askthetachemistryhelp.com and I'll send ya the Excel file.