Oni Dagger can last you until around the middle of Sekigahara. I used a +70 Tatsujin sword from then until I started farming Kaga (level 30 something sword from Kyoto... I think it was a blue and not even a red/gold). Then my Kaga weapons lasted me until the end of Echigo where I then made a Kogarasumaru and a Seven Star Sword which lasted me until after farming pipe 3 weapons. Then again, most players can't manage that so it's probably safe to assume a weapon will last you one full "area", especially as you progress farther and farther through the game.
Also, Blue Moon Blade will probably last you until Echigo and that's it.
Edit: Rhythm says to Edo.