The answer to this is the same answer with any instrument – it depends. Everyone will learn at a different pace, and you shouldn’t get frustrated if it takes you a while. Even if it’s easy to get phased by the number of controls a synth has, just take it one step at a time. You could develop a good level of proficiency in just a few months, and you’ll start having a lot of fun. True mastery of a synth, however, is likely to take years of practice. As long as you keep practicing and having fun, then it doesn’t matter how long it requires.