My bard got all her gear stolen and had to get a new flute - is it a short rest or a long rest to make that a spellcasting focus?

I don't understand the question. A flute is already a spellcasting focus for a bard. You don't have to do anything to it to use it.

You might be thinking atunement is required, which would be a short rest. But that's just for certain magic items.  A focus can be used simply by picking it up.

Great! That's what I was hoping since it didn't specify, but I wanted to be sure - thanks!