> "how long until this is good enough to take over"

Depends on your definition of "Take over"

I suspect there is going to be a lot of layoffs in the call center sector as soon as a customer service LLM company gets spun up with competitive rate for per company fine tune and maintenance cost.
As soon as one company does it the rest will follow swiftly. Leaving a skeleton crew of humans to verify the large changes to accounts whilst everything else is handled automatically by a LLM and speech synthases software.

Same thing will likely happen where any ridged formal structure is, law for example. A lot of stuff happens outside the court room that could be automated and likely with those branches of law that don't hold peoples lives in the balance. (e.g. corporate merges/acquisitions vs criminal trials.)