Hmm! This is tough, and I agree with folks who've said it's very subjective. Some people know that they need an entire script before they start in order to stay on focused, but if I'd tried to create a finished script before I started my webcomic, it would have become a reason to never start.
I think..... planning becomes procrastination when it turns into a discouraging reason that you cannot get started, OR, when it is not relevant -- things that are not moving your process forward.
For the first kind, there's a big difference between "ahhhhh I can't start, I haven't even figured out what medieval cities look like!!" versus "Okay, I'm going to draw some medieval cities so that I can check that off of the Things I Need To Draw For This Story list and get started." When preparation becomes a reason to back off and circle for a bit instead of a step forward towards starting your thing, it's at risk of becoming procrastination.
I think that's something you have to recognise mentally -- it's not how much planning you do, but why you're doing it, that determines whether it's procrastination or planning.
On the other hand, for the second kind, I think there are some folks who just get distracted. Creating an entire timeline for the history of every country of your fantasy world is fun in the same way that drawing endless sparkly anime eyes on your math notes is fun -- it's comfortable and mentally stimulating and feels like creating, but doesn't require you stretch yourself to actually finish something. This isn't bad, if you're having a good time, but it can become a bit of a trap if your dream is to finish a story.