google maps shows 727km from Adelaide CBD to Melbourne CBD, so factor in maybe another 300k's of driving around in Melb or in Adelaide, say 1050k's, your car gets 400kms to tank (hypothetically), fill up your tank when you leave, fill up once on the way there, again in Melbourne, and once on the way back from Melbourne, so around 4 fills. But thats statistically, shitloads of variables