You can usually get through the game if your characters are level 50-55 and you have a decent understanding of the last booses' battle tactics, but this is only if you have been picking up most of the best equipment and abilities, otherwise, you may need to level your characters up a bit further.

Also it is virtually impossible to get through this game too quick if you power level at certain key areas, they say its possible for a decent player to get through the game in 16 hours if you know how, personally I have done it in 10 and a half, but I did have quite a bad migrane later that night.

The best thing to do in the case of FFIX and all the other final fantasy games in fact, is to spend lots more time than you need on the game rather than trying to rush through it. If you rush through the game you don't appreciate it, and if you take your time you do. Plus its also much more satisfying having just completed a completionist playthrough rather than a proffessional playthrough.

The most gameplay time I have ever had on FFIX without wasting time and having completed all of the side quests is 61 hours, but this does depend on some things like your luck in chocobo hot and cold for example.

As I said, completionist playthroughs are much more satisfying than rushthroughs.