Same-domain CMS migration with proper 1:1 redirects: 2-4 weeks of crawl turbulence; 4-8 weeks to full stability. The 523-day stat is domain-to-domain.

Realistic recovery timeline (confidence-labeled):

• Same-domain CMS swap, complete 1:1 redirects, no URL restructure: Plan for 2-4 weeks of crawl turbulence and 4-8 weeks to full stability. (Confidence: medium-high.)
• Same-domain with URL restructure: 4-12 weeks. Google Search Central's site-move-with-url-changes: "A medium-sized website can take a few weeks for most pages to move in our index; larger sites can take longer." (Confidence: medium.)
• Domain change + CMS swap: the 523-day SEJ average ([[sej-892-migrations-523-day-recovery]]) applies. Plan for 6-18 months and tell the client explicitly. (Confidence: high.)

The Mueller "weeks not months" framing has a primary source but it's narrow. The verbatim quote ("maybe a week or two, maybe up to three weeks… one, two, three weeks, something around that range") is from a February 19, 2021 Google Search Central Office Hours, reported by Roger Montti at SEJ. It was specifically about AMP URL crawl settlement after a parked-domain move, not a general statement about CMS migration recovery. The widely-circulated "4-12 weeks for a medium site" framing is agency synthesis, not a Mueller primary source.

What actually causes the "migration penalty":

Per the SEJ study, Numen Technology, and credible practitioner post-mortems, the penalty is execution, not algorithmic. Reproducible failure modes in order of frequency:

1. Incomplete redirect map (missing attachment URLs, pagination, archives).
2. Soft 404s — pages that "exist" on the new site but are empty templates.
3. Schema dropped without replacement.
4. Staging environment accidentally indexed before cutover.
5. Redirects implemented as 302 instead of 301.
6. Redirect chains (old → intermediate → final).

See Migration hidden-killers catalogue — the seven failure modes that appear in every post-mortem for the full list.