I'm planning to apply for EW-3 visa, I would like to know about the time period.

Visas are issued when the priority date is current. Every month, the Department of State publishes the Visa Bulletin which contains cut-off dates for various visa classifications. The applicant's visa becomes available when his or her priority date is earlier than the Visa Bulletin cut-off date. The priority date is set when an application for labor certification is filed on behalf of an employment-based beneficiary or when the immigrant petition is filed (when labor certification is not necessary as in the case of nurses, physical therapists or multinational executives and managers). While priority dates are expected to progress over time, take note that visa retrogression could happen as well, thus, resulting in a longer wait for the visa to become available. Also, visas for certain countries, particularly the Philippines, India, China and Mexico may not be the same as the rest of the world.

Immigrant visas are issued are available based upon priority date (the date the immigrant visa application was received by USCIS). Availability for visas fluctuates based upon demand; higher demand for visas in a particular classification means a longer wait for a visa to be available. Wait times are also effected by your country of citizen as visa availability for citizens of some countries (Mexico, China, India, and the Phillippines) are even more backlogged.

I agree. Review visa bulletin for information on priority movement.