6-18 months depending on how many places you have lived.
Also, how many times you've been arrested, how many times you filed bankruptcy, your relationship with the IRS, your relationships with "colorful" people (family & friends) and so on.
And the older you are - the harder it is. Lots of history to go through.
At Oak Ridge (K-25) there were tons on 18 to 21 year olds strutting around with white hats (white hat is the tag for 'L' or 'Q' cleared personnel) and us old roadies sported orange hats.
I was there 14 months. I never saw white (see above - I touched them all - the guy who interviewed me didn't want to see me again). Another infamous "NukeWorker" was there much longer than I (
they came to see him to interview him (
3 times!!!) and he never saw white either.
Bottom line is - it really all depends how bad they want / need you in.
Look at that Chinese National that pumped nuke secrets back to China. He had a 'Q' - he also must of had scary math or computer skills to dish his traitor ass a clearance.
But me, white boy born in the U.S.A.? "Ya'll go right over there and have a seat on that there microscope boy."