If you have fully submitt the required documents, it take around 2 weeks (the staff told me that i will receive within 4 weeks) to receive their confirmation letter.
The required documents as follow:-
1. Salary proof of u and husband.
2. Proof of Monthly Rental fee
3. Proof of Monthly Mgt fee
4. HKID Card copies of u and husband
5. Copy of birth cert. of your kids
6. Rate etc
For more info, please click http://www.info.gov.hk/swd/html_tc/ser_sec/soc_secu/index.html