What Does Low Grade Squamous Intraepithelial Mean?

hi i found out i have low grade squamous intraepithelial..i know it goes away on its own..but my question is...how would i know how long ive had it? and why hasnt it gone since? how is there a 12 month period on it to see if its gone when we could have it for years and it hasnt left us. im confused