How far apart should your holes be drilled? That will depend entirely on your needs. I have 2 tormatos that are 7.5 feet tall! In order to cover the whole length of the PVC with only 20 feet of Pex, they had to be spaced fairly far apart. On this one they are 12" apart. This is all the taller I would recommend making your tormato.
Typically, I go with about 6 feet above ground and the holes spaced about 8 inches apart.
I also like to space the loops near the bottom closer together (so the bottom hole will be closer to the second to last hole) and try to make these loops smaller than the rest. The tomato plant won't be as bushy down there, and should be trimmed near the bottom anyhow, but there should be good support so the bottom branches do not contact the dirt.
Here is one with 10.5 inch spacing, you can see the loops are wider and have quite a steep slope to them.
Here is one with 6 inch spacing.
It all depends on what your needs are. You may only want a 4 foot tormato. You could probably make one out of only 10 feet of pex. If you do, please send me pictures!