the beauty of that, is that YOU get to decide then. If you need it to be somewhat specific, then take your finished length (whatever it says, or pick one) and divide it by the number of colors you have and how many times you want to repeat it and all that. It really is a very easy pattern and I would be more concerned about getting extremely bored with it than anything else. OR you can google "random stripe generator" and have it tell you how many rows to make it.
It is your blanket...have fun with it!