Hi,
Are you referring to the time as to when you should declare a low and emergency fuel state.
Well different airlines take different views of that and incorporate the same in their policies. A good reference to consider would be with regards to the holding fuel remaining on board.At any time if you find that the EFOB on the MCDU(A320 family) is 1200 KGS(30 mins. holding fuel) you should declare a low fuel state to ATC(PAN call).Likewise if EFOB is 600 KGS(15 mins. holding fuel) you should declare fuel emergency(MAYDAY).
Hope you are never in this state...............cheers
