It depends on your party. If you have a balanced F/?/(RM or WM)/(RM or BM) style party, 25ish is a good time to beat the game. I think the old NES FF1 Strategy Guide recommended level 27. I could be remembering incorrectly. I'll have to look it up when I get home.

[qq=ValiantKnight]The dungeons are 3x as hard as the actual boss themselves. Just because of the endurance factor and lack of healing on Normal origins and original NES.[/qq]

This is true. Usually the hardest part about finally beating the game is making it to Chaos. Thankfully(?) the hard part of the ToF is mostly at the beginning with all the gas and frost dragons. Though for some reason the water floor also tends to hurt me a lot. It's not just having enough healing magic/items, it's also having enough black magic left to kill large enemy groups quickly enough that they don't stand there pounding on you for 10 turns. And the need to save all your FAST till the end.