Multiple Times

[javcs]

Example: Half-dragon->Half-Fiend->Half-dragon.

Especially since half-dragon and half-fiend can both be acquired templates.

You've got the order wrong - half-celestial > half-dragon > half-fiend > half dragon.
Possibly with a half-dragon on the front end to get the alignment requirements out of the way.
Or half-dragon > half-celestial > half-dragon > half-fiend.
All depends on what you want the final type to be.