How long it takes varies from individual to individual, and even the same person at different times of life will need to spend a different amount of effort, and different techniques will work for different people better. Lucid dreaming is a very individualized experience and it depends on a lot of factors including among others which level of awareness you are starting from: someone who goes through life on autopilot in mental fog will most likely take longer than someone who is used to paying attention already, how good your memory is also has an impact, hormones and other chemistry are a factor, how much sleep you get every night and how regular your schedule is, and also your expectation as and intensions and dedication level will influence how quickly you see results. Lucid dreaming takes a special approach to life: you need to go through life with an open mind to question your reality, and that takes some getting used to, and you may need to unlearn some bad habits. Most likely it will not happen over night. That said it is always a good idea to always go to bed expecting this to be the night when you remember to realize that you are dreaming.