Gameplay is pretty straight forward, almost no learning curve there. I do suggest that you read up on what each attribute/perk does though, so you have a rough outline of where you're going with your character.
The learning curve is pretty steep at first. However things are not overly complicated. Learning the basics is fairly easy but figuring everything out will take time. Playing on easy or very easy is very forgiving and might be an option to consider when starting out.
One bit of advise I'd give, get some coffee or other beverage of choice, maybe some snacks, start up the game & save game, pause the game and go to the help section.
The game does a very poor job explaining a lot of things even for people who have played the previous games, the help section is basically like the game's manual and will teach you far more about certain things then what the on-screen prompts tell you.
Other than that, it's a pretty straight forwards game when it comes to basic mechanics, a FPS with RPG elements.
To play and be proficient, meaning you can move around, interact with the world and generally survive: one or two hours tops.
To master combat: will depend a lot on you, but could be anywhere from 3 hours to 30 I'd say. Actually, when you consider the myriad ways to go about combat, that "30" could well be infinity instead.
To get proficient with settlements: I'd say 5 to 15 hours. At least another 10 to master it, but again could be "infinity." Settlements are also completely optional . . .
The more "strategic" and "logistical" aspects of the game (SPECIAL builds, exploration tactics, combat tactics, Perk tree development, relationships with factions, relationships with companions, etc.): all of that is an "emergent" learning process, but it is not like you get punished in the long run as a consequence of being naive about these topics at the outset. It seems a lot of us play a character for a while, then restart once we learn a bit about this more emergent stuff. I don't think many regret that though. Redoing some of the early game stuff is actually kind of fun, at least for the first or second repeat.
I played Fallout 3 many hundreds of hours, FONV nearly as much, Skyrim literally thousands of hours . . . I'm up around 300 hours of play at Fallout 4. It is a hilarious, scary and beguiling game.
This is my story:
I'm completely new to the Fallout franchise, decided to buy the game because oh-so-many folks have recommended it.
I started playing without knowing *anything* of the game and its mechanics. Reached lvl12 (about 2 days + exploring) and restarted the game with a new character on Survival mode (can't play anything else now.. its fun!).
My new character is lvl26 and rising.
Am extremely happy with the game and the purchase!