I have been asked how long it takes to be a good skier, which is pretty much an impossible question to answer. For one thing, everybody improves at a different pace. More crucial though is the question of what is meant by a good skier. However good you are, there is always somebody better. Is an instructor like myself, with an ISIA badge and a few hundred weeks on snow an example of a a good skier? I hope the people I teach would think so, but put me in a ski race and it is apparent that there are lots of skiers out there who are far better (and faster) skiers than I am. To get to a really top level requires investing a huge amount of time, from a fairly young age, as well as having the talent and opportunities and funding and determination to succeed.
If a good skier is someone who can ski black runs competently (short parallel turns for example) you are probably looking at 3 - 10 weeks from complete beginner, depending on natural ability, confidence, determination and amount/quality of instruction. To be competent off piste in any sort of snow conditions takes a good few weeks longer. Getting to an instructor level will take most people tens of weeks of skiing, whilst getting through the top level instructor courses may well take hundreds. Getting into top level competition, where the really good skiers are, will require dedicating a huge part of your life to the sport.
Lynsey Vonn - Getting to this level of skiing takes many years of dedication