Every brokerage that has been burned by a slow follow up writes an SLA. Most of them write one number, fifteen minutes or an hour, apply it to everything, and watch it stop meaning anything within a month.
The number belongs to the stage
Fifteen minutes is right for a new portal enquiry. It is meaningless for a lead in Nurturing that you agreed to call back in March. One clock cannot serve both, and when it tries, people learn to ignore the breach count.
First response and acknowledgement are different
A lead can be assigned to an agent and sit there. The assignment happened; nothing else did. If your only measure is time to first response, an unacknowledged lead looks the same as a lead nobody has got to yet, and management finds out about it a week later.
Separate the two. Count assigned but not acknowledged on its own, because it is the failure mode that a rota or a lead pool is supposed to prevent and usually does not.
Measure dwell time, not just entry
A lead that entered Contacted three weeks ago and has not moved is not a success because someone called it once. Stage dwell time is where the pipeline actually rots.
Make the breach visible where decisions happen
A breach count in a report nobody opens is a breach count nobody acts on. It belongs on the first screen after sign in, next to revenue, because the two are related and the executive team is the audience for both.
If the number cannot be seen from the chair where something can be done about it, it is not a measure. It is a record.