How Long Does a Defect Count Against Me?

[tenacresplus]

How long do defects count for.  From the time they are received, how long do they counted against being a Top Rated Seller for 3 months or for a year or what.  Seems like the new rating system expects almost perfection.  Plus if my numer of sales go down than the percentage of the defects would go up so even though I am not get any more defects I could still lose being a Top Rated Seller anyway.