You'll get so many different responses to this question...in my opinion...
My first helmet (HJC CL-15 Full Face) was dropped after only three weeks. I thought "Great!! I just bought the **** thing!", and when I mentioned it to my wife, she said I better stop by the Shop and pick up a new one...at only $135.00 I wasn't about to let my head (as simple as some may think) be at risk for $135.00. I've heard everything from swapping it after a 1' drop...I've heard the Helmet companys use this as a ploy to sell more helmets...I've heard of all kinds of things, but I will tell you this...
My head and everything in it...is worth more to me then the price of a new helmet...if I couldn't afford one right away, I would simply wait until I could. There is no amount of riding that is worth it to me...If I couldn't wait...too bad. Yeah there are those that say "F*** it"...I ride no matter what...great! Go for it...Tough Guy! I've got a friend that tosses his Full Face Arai on his work bench in the garage every time like it's tupperware. I respect myself much more than that.
So what you have to ask yourself is...Is it worth it...is it worth it to "test" the theory? The "One-Drop-it's-Done" theory...Or are you worth more than the price of a new helmet...?
I love it when I see guys/gals that spend $500+ on a jacket, $300 on leather pants/chaps, $200 on riding boots, $100 on gloves...and $30 on a plastic molded bowl ("Pudding bowl helmet") that I wouldn't wear while riding a bicycle!
When I AutoCross you have to wear a Specific Automotive racing helmet...no Motorcycle helmets...Motorcycle helmets are designed to take a one time impact (intial impact followed by deccellerated impacts)...Racing helmets (Auto) are designed to take multiple impacts (ie. Roll Over)....
http://www.soloracer.com/helmettech.html