Once herpes zoster is triggered, how long does it generally take for the rash to appear?

Usually no trigger: Your question seems to assume a trigger of herpes zoster (shingles) is usually present or can be identified. Not so. Most cases are random without any prior symptoms or obvious trigger. Conditions that increase risk of shingles, such as immune deficiency of various kinds, typically are present for a long time, often years, before an episode of shingles appears. See MD ASAP if you suspect shingles.