It depends almost more on your stats and what you've found than your levels. If you've increased each characters' stats(vigor, speed, magic, etc...) by using espers and have powerful weapons and/or magic like Illumina or Ultima, then the end isn't very hard at all. You basically need one powerful character in each party to carry the workload and you can let the others catch a free ride. Then when you get to the final battle just make sure that the powerful characters are in your party and you'll be okay.

But as a generic answer to your question, 45-60 is a good level for your characters to be at, as it depends upon what you have and their stats.