Hi minghang,
I am assuming you are asking about the time it takes to receive an ITA after submitting the PR application under CEC with a positive LMIA. There are draws every 2 weeks and candidates with CRS scores more than or equal to the cutoff for the draw will receive an ITA. So it basically depends when you submit the application with respect to the draw dates i.e it can be next day to 13 days from the day you submit the application.
If your question is indeed how long you would keep getting an ITA after submitting the application then the answer is just once (assuming you meet the cutoff score of an invitation round), if you accept the invite and continue with the Post ITA processing. Else, you will have to decline the ITA and wait to receive next ITA which entirely depends on your CRS score and the cutoff of the subsequent invitation rounds.
You may go through the following link for further understanding of the Express Entry in general.
http://www.cic.gc.ca/english/express-entry/rounds.asp
http://www.cic.gc.ca/english/resources/tools/perm/express/pool.asp
Best luck for the journey ahead!