They usually send it via priority 2 day mail so you get it in 2 business days. sometimes it takes 7-10 days if it is sent via normal post.
Just in case someone else sees this thread and wants to know, once you receive the notification that the document has been mailed to you, you can call USCIS customer service at 1-800-375-5283 to request the USPS tracking #. Here are the steps:
1. call 1-800-375-5283
2. follow instructions, and enter your receipt #
3. you will get an update on your case, and press 3 to talk to a rep/report a problem
4. a rep will verify some info with you and you have to tell him/her you want to know the USPS tracking #
5. you will be transfer to a level 2 rep, probably verify your info again, then he/she will tell you the USPS tracking #
The level 2 rep i spoke with was much more friendly than the first one...
mine was mailed through Priority Mail 2-Day.
Hope this helps.