Ray Casting

[Nobody]

I was looking up tutorials for learning how to do ray casting. This was the most frequently referenced site:
http://www.permadi.com/tutorial/raycast/
I read it and understand most of it. One quick question though. It has a section about doing walls of various heights, instead of having them all the same. There’s one part that says:

The main drawback of using variable height walls is that the rendering process will be considerably slower. This is because the rays no longer stops when the closest wall is hit. One way to speed this up is to set a visibility distance and simply ignore anything beyond that distance.

For a little more info, this tutorial uses a grid that’s “64x64x64 units” and each time a move is made forwards (or backwards, or left or right) you move 10 units.
So then, what’s a good limit for visibility? If you have a straight number that’s great, or, if you have a formula, that’s even better. Thank you very much for your help.