Usually the evaluation of the abstracts starts after all abstracts have been sent - i.e. after the submission deadline is over. Depending on how many abstracts are there and how many reviewers/what review procedure they apply, they will need a few weeks before coming out with the selection of papers for the conference.
In my experience so far, response comes about 4-12 weeks after submission deadline, it depends a lot on the conference timeplan of course - and also if they plan to print out proceedings during or after the conference.
Best
Wanderingbit