On my SF-86, I stated "I smoked marijuana for a 6 month period my sophmore year of college.  I never smoked again after quitting and I never tried any other drugs."
I wish I hadn't made such an open ended statement.  I'm worried the polygrapher is going to go to town on this.  especially asking me how many times I smoked (I honestly have no idea, probably twice a week)

How can I stick to my statement if he asks me to quantify how many times I smoked?

my plan is to say "sir I did not count, I have absolutely no way of quantifying how many times I smoked, any number I gave would be a complete guess, which I will not do because it is a separate statement from what I said on my sf-86."

Is this an acceptable answer?  Polygraphers feel free to comment too, I'm genuinely interested in what you have to say.

You'll be fine.  You have plenty of time between now and the polygraph.  If it was only a 6-month period, sit down and try to count the number of times.  Did you smoke once a day, once a week?  Come up with the best estimate you can and stick with that estimate.  Let them know it is a close estimate but may not be exact.  Be ready to explain how often you smoked, the reason for it (recreation, parties, stress, depression, peer pressure, etc.), how long since you have done it, and if you get to the background check stage they will find old friends and ask them about your drug/alcohol habits.  Smoking marijuana does not disqualify someone if it was over 1 year ago and infrequent.

Well you are going to have to come up with an estimate.  They will ask you "10 times?  50 times?  100 times?"  If the number is over 100 or too many times to remember, then that will raise a flag.  It means you are probably over the threshold (which they won't tell you what it is), so you'll be eliminated.  So you better come up with some ballpark figure otherwise consider yourself done.

That's right. If you are not sure, they will start throwing ball parks out there to get it narrowed down. They are going to want dates, times, and even who you smoked it with and who you got it from. Keep in mind that they are probing to see if you may have sold it or where it ultimately came from. They will obviously keep an eye on the machine and probe harder or lighter accordingly. Obviously, they are trying to see if there is something underlying here. Not to alarm you but, that's how it may go down for you. At least you have some insight.