For rockets, there is a rule of thumb that says for each kilo of fuel put into orbit, 7 kilos is needed to get it there.
For long and ultra-long flights, it feels intuitive that some very significant proportion of the fuel, perhaps the majority, is needed to carry the fuel needed to complete the flight. InIn other words, imagine that the fuel was weightless and 50,000 litres were needed. ButBut since fuel is not weightless, the requirement is actually 100,000 litres.
Of course there are a lot of variables and it's fairly easy to calculate, but is there a rough and ready rule of thumb?