It's been years since I've taken benzos or anything like that. In the case of anxiety, the med is best utilized at the first signs/symptoms of a panic attack or extended period of heavy anxiety.

When Haldol was just my auxiliary PRN med for my more serious super crazy fun times, I would being dosing once I had failed to achieve sleep in at least a couple days and other issues were beginning to arise (not eating, self-destructive actions or impulses, worsening psychosis, etc). Now it's just a primary med for me, which I'm permitted to increase dosing on if/when necessary.

tl;dr - You just kinda learn after awhile.