On the game jam stuff, remember those guys didn’t just download the engine a few days prior and go “right, I’m going to build this super awesome game and there’s gonna be sharks with frikkin laser beams”, chances are they have some experience and they’re able to churn out that stuff pretty quick because of it. If you’re at that stage, your very basic prototype (depending on your criteria, i.e. when do we check the box and say we’re done?) could be in a few days/weeks. On the other hand I’ve seen people measure success against products of game jams and drop out.
I came from another engine, there was still a learning curve despite core concepts being the same. To clarify I haven’t shipped any entertainment product, with UE or otherwise as I was in the “serious games” sector working for a very large organisation where we weren’t the core business for almost 10 years. I am working on a solo project in UE4 after a few throw away projects to learn the engine and was until recently still using the original blue default character, but any timeline I could give wouldn’t help as I have a full time job and two kids.