"...ten years during which we can only guess at what Lord Voldemort was doing…"
--Albus Dumbledore (HBP20)
After the death of Hepzibah Smith, Riddle vanishes for ten years. He travels far and wide, sinking deeply into the Dark Arts, consorting with the very worst of wizard-kind. He undergoes so many dangerous, magical transformations that when he resurfaces years later as Lord Voldemort, he is barely recognizable.
Tom Riddle came to Hogwarts ten years after the murder of Hepzibah Smith, supposedly to ask for a teaching post. Since Dumbledore became Headmaster sometime after 1965 and before 1970, Riddle's visit would have been during those years. The murder, then, took place in the mid-1950s and Riddle, who left school in 1945, worked for Borgin and Burkes for around ten years.