Karma came from a culture that believes in reincarnation. It is payback for your behavior in past lives. "Instant karma" is therefore payback for your behavior in your current lifetime, without an intervening turn of the samsara cycle.

If you kicked a puppy right before you died, and got slapped too hard just as you were born into your next life, that's regular karma. If you stole someone's car for a joyride when you were 16, and your car was stolen by a joyrider when you were 90, that's instant karma. The number of intervening years is not the important factor, but the number of intervening rebirths.