In order to get dreads, your hair needs to be at least about an eighth of an inch to maybe an inch long. The reason why is because it has to be long enough to at least coil once and an eighth of an inch, it should do just that. Most people try to wait until they get about two or three inches, because you also want to consider the look when you're first going into dreads, and however long your hair is, it might look a little shorter.
So that may cause you to wait until it gets to be about three to five inches long, but if you're eager to get started, and your hair is very short, as long as it's longer than an eighth of an inch you can get started in to your dreadlock journey. On the other end of the scope if your hair is really long and you want to go into dreads, it really doesn't matter how long it is.
You can have between 6 to 12 inches of length of hair and still be able to get dreadlocks and it just depends on what type of method. There's many methods to choose from. It depends if you decide to do it naturally or to use a method that's going to force it to lock more quickly, but the length isn't the issue, as long as you have an eighth of an inch, you can definitely have dreadlocks.