It might be easier if the data is in chronological order, but if you keep it like it is then you could put this formula in F2 for example:
=IF(TEXT(A2+B2-1/24/60,"hh:mm")=TEXT(A3+B3,"hh:mm"),"1 min","15 min")
then copy it down. It agrees with your hard-coded increments.
Hope this helps.
Pete
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