I've only had my Model 3 since late March, but my observation is that a new weekly build (like the update from 2019.16.x to 2019.20.x) usually goes out to just a few cars at first, and then a few days later it gets blasted out to a large number of vehicles, often with a sub-build version upgrade (like 2019.20.2 going out instead of 2019.20.1 -- but I'm not sure if that was the exact sequence for that version). This pattern suggests that Tesla tests to new release on a few cars, makes tweaks based on the results, and then delivers the latest version to most of its fleet.
The 2019.24.x builds first began to appear over a week ago, but I don't recall the exact date. It seems to have been pushed more to Model S/X vehicles than to Model 3s, especially at first. After an initial 2019.24.1 release, several other variants, now up to 2019.24.4, have appeared on TeslaFi, and the whole process has struck me as a bit slower than previous updates, although I haven't been tracking exact dates, so I might be wrong about that. Overall, the sluggishness and high sub-version build number (now .4) before a big release looks to me like Tesla may be running into bugs in the early releases, and this is slowing it down. So far, the big feature I've heard will be in this version is CHAdeMO support for Model 3, but the bigger rollout to date to Model S/X owners suggests there's something more important (whether user-visible or not) for those cars. That could also be part of why it's taking so long -- if there's some tricky or critical change (say, something that affects how the Model S/X motors are controlled), Tesla might be rolling this one out in a very cautious way. These two hypotheses (bugs and caution because of a big or important change) are not mutually exclusive, and may in fact interact with one another.
Most of the preceding paragraph, of course, is very speculative, so take it all with a grain of salt.