In most states landlords can only charge up to 5% of the rent for late fees.
So for example if your monthly rent payment was $250.00 your late fee would be $12.50
I have seen some landlords charge up to 10% late fee but in some places that can be illegal and may not be enforceable.
Also the landlord should not charge the late fee until you're at least 3 days past due on the rent so the late fee shouldn't happen right away.
Another thing is if you signed a rental agreement and the rental agreement or lease doesn't say anything about a late fee then the landlord cannot legally force you to pay a late fee for being late on the rent.