There are not specific guidelines for this but as a renter, it would be reasonable to expect carpets to be in well maintained and clean condition. Anything short of this should be addressed prior to signing a lease agreement.
I'm not aware that there are any specific rules for how often a landlord must change the carpet. If you are a renter, you are entitled to a safe place to live, free of pests and with functional appliances. Page 8 of the attached form may be of some help.