Generally speaking, day trading can be learned in a period of time lasting from a few weeks to months (or never).
Off course, this depends a lot on the individual that wants to learn. Some people take a lot longer (more than six months) to even learn basic strategies and how to place orders. Some traders pick things up rather quickly, speeding up the learning process.
If an individual is dedicated and spends a lot of time applying the trading strategies he is taught (assuming the strategies are good) on a simulator or demo, it is probable that he might be trading within three months.
Even though complete mastery of trading can take a lot longer, there is no reason why a new, serious trader should not begin seeing some fruit before that time. If you are a person that cannot stomach trading, an automated system might be another option for you.
To start learning about day trading, start by reading my 7 steps.