How often does NetSuite update the index used by Global Search?

Shouldn't it be immediate? Are you saving a record, then it takes time until it shows up in the search?

I seems to be at least a few minutes. In this case, I was actually removing something from appearing in Global Search (I unticked the Global Search checkbox on a custom field)

Hmm, I don't have the answer that you are looking for. But AFAIK, indexes get updated automatically by the database, but several factors could delay the process, like how many updates are happing on the database at the same time, and maybe there are some factors that only the internal netsuite team know, and not published.

Lately seeing lots of caching issues after the last release, so it can effect the global search indexing