Well, their intention, stated in the S19 blog is for seasons to last roughly three months. That means, as S19 started on 22nd November, the end date should be around Sunday 23rd of February. That’s four weeks away. They also stated that they wanted to have a two week long PTR, which they haven’t announced the start date for yet. Even if they announce the PTR today, and patch it in on maintenance Tuesday tomorrow, that means it would run until 11th February.
Now, they also say that they’ll give at least a fortnight’s notice of the end date of a season. If they were going to stick to 23rd February (three month season duration) that means they’d have to confirm the end date by 9th February, i.e. they’d have to confirm the end date whilst they’re still in the middle of the PTR without knowing if something major might be found in testing that would have to push back the season end date.
TL;DR - if PTR isn’t announced today, S19’s very likely to be extended. That’s the timeline.