Is this a normal thing for Polish people to say? It sounds like a politely aggressive way to get guests out of your home, the counterpart of "O Boże, idźcie już do domu, błagam was."
(Honestly though, I can't think of any other reason to say this.) :D
Teachers telling their students they need to focus for a moment more? The beginning of the end of some lecture, going on how you should carry on with your life, project, etc. ( I am pretty sure I've heard priests saying this during "kazanie". )
Ahh, I think I see now. However, it still seems awkwardly worded as an English sentence. This could partially be down to the abundance of tenses or structural differences compared to Polish. (The first on your list would most likely be "You will be going home in a minute!")
The second and third examples I could see as stated though, but it seems odd as a standalone sentence, (in English anyway.)
The only situation where I can imagine this sentence being used is parents trying to get their children to behave. "[If you don't start behaving yourselves,] in a moment you will go home.". The part in the square bracket can be left out to be inferred from the context.
As a parent (living in US, but no claim to be an English expert!), the only time I would use that expression with my children would be when I would want to encourage them to finish something (much like the teacher example given by "immery"). And yes, it would have to be in a context, not by itself. For example, when we shop and MY kids get bored and keep asking: "Are we done?...", I would use the expression: "In a minute we will go home" with the implicit understanding that they will get to do whatever they want (once home)... I never use the expression in a negative sense BUT that is me! HTH.