Beginner to Capt?
The answers to your questions depend primarily on how motivated you are to learn, and how quickly you are able to grasp sailing and other concepts. I once taught a husband and wife to sail in an hour, they bought their first boat within the next week, and by the end of the sailing season, they were pretty good inland lake sailors. He was an engineer, and he understood and learned technical concepts easily.
If you complete the basic keelboat course, you should be able to sail in sheltered waters where there are no particularly difficult navigational challenges. If you have common sense and a little sailing experience, you should be able to sail a 40 footer in sheltered waters as readily as a 20 footer. For many years, when charterers checked out a sailor to see if he was competent to bareboat charter a 40 footer, they were only concerned with whether he could maneuver the boat under power, around the docks. They didn''t check to see whether he knew how to raise and trim the sails. In many ways, big boats are easier to handle than small ones, because they are not so easily blown around by the wind while docking, and they are usually stiffer than smaller boats.
I remember reading of someone who crossed the Atlantic solo within a year after learning to sail, but most people would not be able to learn that quickly. You have to learn to sail, and you need to gain enough experience to strengthen your judgment. You also need to learn about navigation, meteorology, storm sailing techniques, first aid, and all sorts of other subjects, and you need to be an extraordinarily resourceful and self-reliant person.
Most sailors will never cross the Atlantic. Sailing along the coasts and inland lakes is a very satisfying way to spend a lifetime of sailing, without having to endure all the hardships that go along with bluewater sailing.