suggestion

can u suggest how many times should I run SFC scan now command in a month?

Why would you do anything of the sort? SFC scan is a troubleshooting tool to be used if you actually observe any wierdness on your computer that may be related to corrupted system files. Just enjoy the computing, stop obsessing about the internals.

In the past some how those files got corrupted and could not replace them.

bagavan said:

In the past some how those files got corrupted and could not replace them.

Your HDD could have run into problems causing corruption

"if it ain't broke, don't fix it"

sfc can be run as often as you like.
I don't think that a sfc run can do any harm to the OS.

The only thing to consider is that running sfc, if there are no errors, is waste of time and computer resources.
And the HD has a working hours limit...

Earlier I was also paranoic about the health of my computer an OS and have run sfc from time to time just to get the message that sfc did not find any errors!

Windows 7 seems pretty stable and has a number of selfcheck tasks running by default.