I'm coming very close to being 100% eligible for my security clearness but some of those questions would be tricky for me right now.
I've often wondered if I could round 8 years up to 10 to pass a polygraph.
HOWEVER: law enforcement agencies are quite a different story and tend to be much stricter in their requirements. Some, like the FBI, have public drug qualifier policies you will need to meet [1] and there is no wiggle room (1 day short of 10 years and you're 100% ineligible). Some agencies do not publish this policy (i.e. Secret Service) and you have to guess whether you're eligible.
The most important thing to remember is that whatever you admit on these forms will be on record for review by any agency for the rest of your life. DO NOT try to give extract or estimated numbers of usage on your SF-86 unless they are exceedingly low numbers. You can be honest with your background investigator during the followup but there's no reason to commit those things to writing on a form that's likely to be easily accessible later on.
I was pretty honest about my drug use. The way they phrase the questions sometimes leaves you with a bit of doubt, though. Like, "How many times did you use this drug?" I'm not sure who keeps a really diligent record of how many times they do drugs at parties.
But, yeah, I tried to be as accurate as possible, modulo that doubt about how possible it is to accurately answer these questions.
I have no idea what the rules are now.