Just curious: about how many hours of gameplay will it take for a first-timer to get through the game once?

Depends how fast you could read i guess :p

It took me about 13 months, but I seem to be much slower at these games than some of the speed freaks around here. Plus I got nailed at every turn and it took me ages to work out how to get past several of the tough encounters (e.g. Palisades, Bridge fight, Aurilite rangers, Chult Guardian etc etc etc).