In jurassic age companies you can get up to speed quicker, there is less to wrap your head around and the onsite staff tend to use a simpler approach (manual methods etc).

If you think you are "up to speed" on a large company a week or so you are usually so incompetent you don't know what you don't know.

You can be contributing code in a week or less, but if you're getting fully up to speed on any moderately large system in under 3 months, you probably should take breaks for meals and sleep more. Or you're not getting fully up to speed and are just ignorant. It could be less for a trivial or entirely greenfield system, but then you're worrying about other things like requirements/constraints/etc.

Unless you stick to a very niche domain that you know well, there will be a massive amount of new information to learn. It takes many months to absorb it all.

It depends on the domain. If it's something complicated it might take a while to figure out the business logic of the system, especially if it's something large like an enterprise system. Also, I think a better way of putting it is that it might take that long for them to have a net positive effect on the org. As in they are contributing with minimal help from the rest of the team.

Depends what you define as up to speed