How long does it take to become network security engineer?

Answer 1

There is no definite answer for this question. It depends on your level of interest and ability to absorb working security practices (and apply them effectively to secure any system).

You can go about trying to get certified in this field; Look into:

CCSP - Cisco Certified Security Professional

CISSP - Certified Information Systems Security Professional

There are specializations still as a Network Security Engineer:

Have a look at the GIAC Certifications: http://www.giac.org/certifications/

You can start by knocking down one of the most basic certifications:

CompTIA Security+