This is dependent upon the state in which you reside, as Aicha has said, and it is governed in part by whether or not there are city ordinances that govern late fee administration. Even if there are no ordinances, a judge may determine what is "excessive".
When not otherwise stated by ordinance, the rule or guidelines that judges use in the US is that the amount of the fee must be reasonably related to the amount it costs your landlord to deal with your lateness. If you believe you are being charged excessively for late fees, it is best to consult an attorney, who will be familiar with case law and local statutory limitations for your state and city.
Here is another resource for you if you are in the US; it discusses tenant rights and remedies. Good luck:
US Department of Housing and Urban Development: Tenant Rights and Resources
When not otherwise stated by ordinance, the rule or guidelines that judges use in the US is that the amount of the fee must be reasonably related to the amount it costs your landlord to deal with your lateness. If you believe you are being charged excessively for late fees, it is best to consult an attorney, who will be familiar with case law and local statutory limitations for your state and city.
Here is another resource for you if you are in the US; it discusses tenant rights and remedies. Good luck:
US Department of Housing and Urban Development: Tenant Rights and Resources