How often does carpet need to be replaced in apartments in Illinois?

We have lived in the same apartment for 3 years in Schaumburg, Illinois and we will be moving out in November. The carpet is very light and after 3 years it has picked up so many stains and discolorations that will not come out. We don’t want to lose our security deposit over the carpet having to be replaced. If you have an answer for me please source where you found it so I can use that. The management at the complex are not honest with anyone and I will need to have something to back this up with.