What is the normal lenght of time for a government security clearance check?

I have applied to Perot Systems government services division, I have passed all phone interviews and the next step in the hiring process is in office interview and government security clearance check. How long does a usual government security clearance check take?

Depends on the level of clearance needed. For SECRET, about 6 months. TOP SECRET and higher can take 18 months or more.

If you have non-US citizen family members, extensive foreign travel or residence, it can delay the process further as they try to run investigations overseas.

If none of this applies, you've never been in trouble with the law, no drugs, etc., the process could move more quickly, though there is a backlog of investigations right now, so 6 months is probably a good bet.