1.5 years is not 6 months. 1.5 year is fast and possible if you have a sqeeky clean, background as i've alread said. the average is 2-5 years depending on your background (5 is rather long and rare but i have seen it). if you have a divorce it takes longer, if you have someone in your family who is a foreign national it takes longer, if you've traveled overseas or had credit issues it takes longer, if you had got cited for underage drinking ever it takes longer. if you have a degree in a field in which there is a have a critical need they will move a little faster. a full clearance does not just mean Top Secret.
not going to argue they can tell you whatever they want i've been doing this work for 9 years just quit in june for law school. i have had a TS/SCI full scope with full lifestyle polygraph since 1998 and was a Marine Corps veteran. the average applicant takes doesn't go from application to Quantico in 1.5 years.