While they're short compared to modern RPGs that can take upwards of 100 hours, I've always found the earlier Final Fantasy games longer on average than their contemporaries. I'm too lazy to dig out my copies to check my completion times, but I know Final Fantasy II (the PS1 version on Final Fantasy Origins) took me over 30 hours to beat, and the DS remake of Final Fantasy III took me around 50. Maybe the Famicom versions move at a brisker pace, but I can't see them being sub-20-hour experiences for me, let alone sub-10-hour. I played the NES version of the first Final Fantasy, and as best as I can recall, the game doesn't even have an in-game clock, but I'm sure that game took me at least 20 hours.

As for Dragon Quest, the grinding necessary to progress in the NES version of the first game adds to the clock, but I know I blew through the GBC version, where you get more EXP and cash. I very well may have spent fewer than 10 hours on that version. Granted, I was going into it with the experience of having already played the game on NES, and I was using the maps from the NES version to quickly progress through the dungeons. I only ever played through Dragon Quest II via the GBC compilation. Not sure exactly how long that one took me, but I know it was a good bit longer than the first. At least 15-20 hours, I'm sure.