28 days from the day on which the notice was served.
Minor correction, it's WITHIN 28 days, so a response received by the Police on the 28th day after the S172 is received by them one day late (day of service counts as Day1).
At the OP. FRankly you've made a hash of posting this by jumping on one erroneous assumption and not telling us the full story, so suggest you lay out a detailed timeline of what happened when and how you responded and any explanation so
31/1/24 Alleged speeding offence of XXmph in YYmph limit
DD/2/24 Received NIP/S172 request (logically 07/02 deemed served - could have arrived a day earlier - to give S172 offence date of 06/03)
DD/M/24 replied to S172 request with.........
Some hints for that reply
What was your S172 response? Why was/do you think your first response unsatisfactory? What EXACTLY did you respond to that with? How quickly?
Crucially did you identify ONE driver unequivocally in the reply or either say 'dunno', or give a list of names or add a caveat such as 'I'm pretty sure it was....'?
Are you dual charged with both the S172 and Speeding?