I'd say the average playtime through an FF's story is 30-40 hours.
Like you will undoubtedly find out, people's opinions on FF differ greatly. FF7 is thought to be remarkable by many because at the time it was so revolutionary. It's still a great game today but the game's graphics don't look so great today compared to how they used to.
Critically many people point to FF6 and argue that it's actually hands down the best FF game in the series so I'd recommend you try that one out too.
Then we have FF8, which has gotten a mixed reception. It's graphics hold out very well still today and it's emphasis on realism paid off. Some people love the game, it's characters and story while others absolutely hate it at the same time. I recommend you try the game out and find out on which side you belong. I personally love the game.
FF9 is a kind of back to the roots and basics kind of a game. While FF7 and FF8 aimed for a more modern setting, FF9 actually went back to the NES and SNES eras and had an older, medieval/17th century kind of a vibe. It's a very decent game and it's main goal was to capture all the classical aspects which defined a Final Fantasy game, including all the faults of the series too. Nevertheless, it has gotten a great critical reception with many people thinking of it as the best in the series. The game's creators often say that this one is their favorite.
FF10 is a kind of a change from the norm, appearing on the PS2. It's noticeably more linear and it doesn't include things like a world map and some other classic FF aspects and it includes voice acting for the first time, albeit kinda bad voice acting. Otherwise it's a great game, filled with great characters and an interesting and enjoyable story. Many people like to think of this one as the best in the series, though that's often because this game (like FF7 for others) was the first FF title for many people.
It's these games (along with VII) which have been considered to be the best in the series. I recommend you start with any one of those, since I think they're all great in their own way. Have fun!