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