A319 with CFM56-5B engines:
Using In-Cruise Quick Check table of the FCOM:
2000nm to go, when you want to cruise FL370 but can only fly FL310, you'll burn 9820kg instead of 9045kg. That is 8.6% more.
If you have only 1000nm left, it is basically same percentage.
Long range cruise assumed, that is, the previous statements on how to adapt true airspeed are applied. Close to 9% is quite a lot and shows how altitude-hungry turbofans are. The more the better.