You can fly as high as your mind can make you.

Who is to say gravity even has to exist in your dreams?

How did you learn to fly? Other peoples stories.

Also, just think about this

As mentioned, at first you’ll likely find it difficult to remain in your dream upon becoming lucid. If the world around you suddenly starts to fade, or you inexplicably sense that your dream is ending, or you even feel that your dream consciousness is thinning, there is a technique you can try to salvage your dream: dream spinning. Dream spinning is extremely simple and consists of you spinning on the spot like you would have as a child. That may sound very silly, but the technique is extremely reliable. When you stop spinning you’ll likely find that the dream clarity has returned, and perhaps your surroundings will have changed as well. In fact, if you focus on changing the setting into something else while spinning, it is very likely you will find yourself in your desired environment after you stop spinning. Note that since it is likely your dream will completely change upon completion of dream spinning, you may lose lucidity. Thus, it may be wise to also remind yourself that you are dreaming while spinning. Also, verbal commands can be beneficial when trying to prevent your dream from ending. Of course dream spinning and verbal commands are tools for controlling your dreams, which we’ll get into next.
I took that from the FAQ... dream spinning. As for all I know just some random guy just said that spinning in your dreams is a way to maintain your clarity. That sentiment spread because no one contested him and some people tried it and it worked for them.

Our reality is constructed on what society says. Some guy said dream spinning works, it did for him, he told other people and it worked for them. This isn't advanced stuff, it is known as the placebo effect. I know I sound like a broken record but it is what I believe, it is my belief.

read about it: http://en.wikipedia.org/wiki/Placebo

I'll take a quote from the page.

The placebo effect occurs when a patient takes an inert substance (a "sugar pill") in conjunction with the suggestion from an authority figure that the pill will aid in healing and the patient’s condition improves. This effect has been known for years.
A doctor lies to a patient but it improves the patients condition, a chemically inert pill healing someone.

If you believe dream spinning will increase your lucidity then it will.

It takes blind faith, honestly. And I'm a chemistry major in University right now, in the waking life I used believe blind faith is the dumbest thing ever. As a lucid dreamer I actually believe religion is a crock but it is not my place to tell people they're beliefs are wrong. I used to be that type of person, the atheist who firmly believed that religion was a mental prison and my duty as a human was to free the religious. I really hate quoting the Matrix but it has some great quotes, "ignorance is bliss." Also what's great about the Matrix is how do we know that scenario isn't true?

I believe that beliefs are personal and people should come to their beliefs on their own.

What am I getting at that with all this?

You set the boundary, you fly like you're swimming. How do I fly? Well I can show you how I fly(note, I took a 30 minute break inadvertently, sorry)

I fly like Super Man, but when I want to go crazy speed I do this: http://youtube.com/watch?v=PxMcNCa1M-Y 0:55 and 2:09

Dragon Ball Z was my favorite show as a kid, at 10 to like 13. The first lucid dream I ever had where I didn't awake after becoming lucid I flew like this. I can can actually show you that dream entry if ld4all is still up because I posted there back then. But anyway, this is how I fly. If I want to go fast I 'power up.' There is an example in the video I posted but it's the first time Goku went SSJ and that would take some technical explanation. At 2:09 Goku would fly so fast that you can't even see him move. That is how the show would show someone is on a different level from someone else and the fight isn't fair, here is a perfect example... I'm sorry I"m just trying to make meaning for something that may not have any meaning for you.

http://youtube.com/watch?v=-AYKQqM0jT4
This video shows what I mean about moving so fast that he doesn't appear to move yet later in the video he is fighting someone else and they are on an even playing field so you can see them fight. The big guy with red hair beat the shit out of everyone and all hope was lost and then Goku showed up and did what he did in this video. As a kid it I was giddy man when I saw this. The season actually ended its run on UPN after the guy with red hair died and we had to wait like a year for Cartoon Network to pick up the rest of the shows. For some perspective this was episode 52, there were 293 episodes of Dragon Ball Z but I stopped watching after a while because it really was just more of the same after Frieza(the white/purple) alien guy died... because you know... it was just believable in a sense that Frieza was the ultimate evil from the begining of the show. There were tons of people who were considered villians in the show but they all stemmed from Frieza, when Frieza died and more powerful and powerful things came out it just lots its charm for me... I'm rambling, I'm sorry. One quick note, if you see the bag on Goku's belt right when he gets off the ship... I still use that today but it doesn't pertain to flying and it actually has a different effect on me now, but that doesn't pertain to flying.

http://youtube.com/watch?v=XbGo0zH6hX4

That video has a plethora of examples of people powering up, if you watched the show. It would make sense. Though the BEST example of what I mean is at the 50 second mark of this video.

It is what worked for me though.

Sorry for this wall of text, I'll address the acutual question. How high can I fly? I can fly into what I call the black abyss of no escape that usually results in waking... it is my nightmare, not out of fear but because there is no escape. I basically leave earth, leave the solar system, leave the milky way, I get so far away from the universe as I know it that I can't see it anymore and it is only me and my thoughts... it is my hell.. I don't have nightmares any more but I would consider this a nightmare. I don't fear the black abyss, when I get there I try to get some where... but I can't.

This actually brings me to another topic but I won't get into... the white room and the black room from my dreams... but I won't get into it. I'll just summarize it with this: The white room is the exact opposite of the black room.