No idea how soon. If I had to make an educated guess I’d say by tomorrow they’ll know the ratings for todays show.
The ratings I post are usually on a two-week lag.
What happens is that (with the exception of prime time network shows, which are measured daily) Nielsen measures audience over the course of a week, Sunday through Saturday, and then takes a few days to release the data. Thus, Sunday shows at the beginning of the week usually take 10 days for the data to be released.