This question is being asked by a lot of folks, here is the answer
- It is NOT REQUIRED to file a AC21 letter with the USCIS

In fact the law and the memo's have said nothing about informing the USCIS. However we on this site RECOMMEND filing a letter.

See the template for the letter on this site, you can send one yourself. If you are changing or appointing a lawyer the we recommend send this letter with the G28 "Change of Attorney" form to insure the letter gets to your file.

http://ac21portability.com/ac21etter.pdf

Remember, there have been many instances where despite filing the letter people got NOID's but in most cases they filed MTR's and their cases were reopened and approved.

Please read our template and instructions to insure best results
Good luck and thanks

Is there a time frame within which this should be done? i.e. submit the AC21 letter?

Since it is not a requirement to send a letter, i don't believe there is any timeframe. However if you are planning to send one then you should do it as soon as possible when you change the job and can provide the new employment letter and evaluation letter from your employer. Its best of course to keep copies of everything especially the mailing and receipt proof.

Attorney at Law Ron Gotcher explains in great details that it's a bad idea to notify the USCIS that you ported to a new job. He says "In my experience, only bad things come from volunteering information to the CIS. I've never seen anyone benefit from doing so."

You can read the entire post at http://www.immigration-information.com/forums/i-140-job-portability/7028-should-i-notify-the-cis-that-i-ported-to-a-new-job.html