Lesson learned too late.

First, when you move in, walk through the apt with the landlord (or apt mgr) and make a not of any issues on the lease: damaged or dirty walls, old or stained carpets, broken appliances, etc. Next, take pictures of anything that is not in order.

When you move out, take pictures of the entire place to show the condition when you moved out.

Finally, if there is anything withheld from the security, they are required to provide you with an itemized listing. How long you lived in a place only matter in connection to security deposits in that there are allowances granted for normal wear and tear. A person who has lived in a place for 10 years will have more leeway for wear and tear than someone who only lived in a place for 1 year.

So the first thing to do is to ask your former landlord for an itemized listing of what the deductions are for. If the carpet is dirty then the charge should be for cleaning or replacing it. Otherwise, check with an attorney to see what your options are if you disagree with the charges.