Your lease must provide you with the right to late fees. Even if it does, you may have waived yoru right by continually accepting rent without charging late fees. Otheriwse, it's enforceable with a 7-day notice to tenant.
What kind of agreement do you have?
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Typically these terms would need to be in the lease but you do need to be careful as to whether you have waived any rights to collect. I would suggest speaking with an attorney and letting them review your lease.