Final Fantasy Legend 1 on Gameboy took me about 6 hours the first time I played it. However, I had already played and beaten FFL 2 and FFL 3 a couple of times each.

Secret of Evermore for SNES took less than 10 hours the first time through.

Legend of Zelda 1 on NES took less than 5 hours the first time through.

Of course, my first time playing any of these wasn't until I was in my 20s. I imagine that they would have taken much longer if I had bought them in my youth when I had much less RPG experience.