i have never paid attention when any of my sites got a number. i have some sites that just got the week avg.
if you go to http://www.alexa.com/data/details/traffic_details?url=http://www.yoursite.com you'll see " Traffic Rank for yoursite.com: 68,340" on a new site it will show: "Traffic Rank for : No Data" thats the number that ad companies show for an alexa rank. i didn't know that. i don't know what that does. i just submitted one my site and i got: i guess that answers my question. LOL btw,one of my sites that is a week old, has a week avg. of less then 500k.
I never did anything to get listed on Alexa, but I'm on there. I used to think it was cool, but I'm finding it more and more useless every day, as there are so many ways that the ranking can be manipulated.
You get your score as soon as you get enough people visiting your website with Alexa toolbar installed.