Dave Elliott: Depends on how much traffic you get. But even high traffic sites id want was a couple of weeks data to make a decision....unless it drops of a genuine cliff.
Micah Fisher-Kirshner: I feel like 2 weeks max (obviously depending on one`s conversion timeline) is the way to go as that`s a long wait for potentially lower conversation numbers
Michael Stricker: Any add-ons or plug-ins would be suspect after such reskin. Schema. Pagination. Breadcrumbs. Auto-generated Links to relevant or popular content. As Benj said, sliders, pop-ups and persistent share tools. But scroll-triggered features and such.
Doc Sheldon: Others have already mentioned the obvious stuff, but a few that have bitten me are lost canonicals, screwed up pagination, broken deferred loading and messed-up CSS. Were they running a child theme before the change? If so, you can probably capture a lot of the customization from a copy of that, so you won`t have to rewrite it all.
Michael Martinez: Look for "nofollow" attributes on internal links and hidden or unauthorized outbound links.