Well it is f2p so I would suggest checking it out first to see if you are okay with the mechanics, graphics, etc (and if you are patient you can do the whole story entirely as a f2p). If you like it and want to get serious about doing the story quests I would suggest buying a month of premium access makes the main game quite easy to breeze through, permanently unlocks the add-ons (if you are interested in them, I would say they are worth playing at least once) and simply enjoy the stories. And once the subscription ends you get "preferred" status which comes a number of quality of life improvements and as said lets access the add-ons. There is actually a nifty reddit page that summarizes all the differences between the various modes so check it out. In regards to time - I never played the game seriously as a f2p (I bought it at the start and generally buy once a year a month of subscription to check the updates etc.) but as a premium player you can certainly do a "basic" storyline over a long weekend (I think that ~25 hours is a good ballpark for the basic class story playthrough and I would add another 5-10 if you check out the "side" quests and so some (solo) flashpoints) and maybe the same if you want to do all the add-ons but I am far less certain about that since it has been quite some time since I last did those.