Here is a rule of thumb I use on the A320;
Lets say we're at FL310 and want to climb to FL350. Take a look at the Max Alt. In this example it reads 344 so we would have to wait for it to read 350 before climbing. The difference in this case is 600 ft. For each 100 feet we must wait 8 mins and lose 400kg of fuel (or weight). So in this example we would have to wait 48 mins (6x8) and burn approx 2400 kg (400x6)of fuel.