How high can I fly?
In Second Life, there's no effective height limit to flying. Unaided, an avatar can reach approximately 200 vertical meters, but if you've got a flight assist script, you can continue to propel yourself upwards to millions, even billions of meters. Some Residents in search of thrills and discovery or who are just plain bored have performed such experiments, and there's much history to setting flight height records in Second Life.
However, do note, as Andrew Linden explains:
... there is a technical reason for not allowing arbitrarily high builds relating to initial balances of the physics engine settings. Right now the build limit is 768 meters. Above that physical collisions don't have the same resolution or correctness.
Related, you'll notice when you fly very, very high, your avatar will start to appear crazily deformed, the skyline will warp, and you'll witness other oddities. These apparent changes will have no permanent harm on your experience once you're back on the ground, but it's good to be prepared. Actual experiences from Residents include: