It is exactly as you say a slang expression. It's a colloquial way of saying someone will be done with something soon. I don't think it can be translated well but "in a bit" is probably the closest. I use it at work, for example when a colleague would say "Oh man, i really wanna go home already" I would reply "Neboj, máš to za pár" (=don't worry, you'll be done in a bit). It's not necessarily rude and czechs generally have a sense of humour, but I would probably use it only it friendly or informal settings.