The answer is it depends. If you have max-leveled well-geared friends to power level and run you through all the missions then I'd say it could probably be done in about 100-200 hours if everyone knows exactly what to do. I don't believe there's level requirements for the missions (Need to get at least level 20 to do the chocobo quest unless everyone's willing to keep you alive while you walk the whole way)
Also note that I'm taking a very conservative definition of "main story" to mean finishing your nation's mission chain, which only accounts for really about 5-10% of the story if we take not including expansions literally. With the expansions you're now talking about maybe 3-5% of the game's story. If that was all you did you would be missing out on tons of stuff.
Realistically speaking unless you have some really dedicated friends this is all theoretical because it's really hard to get a group of people together to run some low level person through the game.
Doing it normally with just people in your level range, I would expect no less than 1000 hours.
Additionally it's not really realistic to say not including expansions because I don't think you can even purchase a copy without them. Last I checked (quit the game about 2 years ago) every time they release a new expansion they bundle up all the previous expansions together and sell it for the price of the original game.
I played for 5+ years with close to 1 year of gametime and I still never finished all the main nation missions because finding people with time to help with old content is somewhat difficult. For the later missions you'd probably want to allocate at least a 3-5 hour block per mission where everyone's ready to go. Literally estimating time to do something in FF11 is like estimating software development. Take your best guess, and double it.
Some things might be easier now than when I played though, and with the new level caps you might only need 2-3 max level friends with a lot of free time instead of 5+ but overall, it's not really a question that has a direct answer.