As it turned out, Gordon Cooper did stay in orbit for a full day, plus ten more hours. That made him the most experienced astronaut in the world, the first man to sleep in space, and the first man to eat a meal in space. The Mercury spacecraft started to fail system by system as the flight continued to orbit the Earth. By the time Cooper was ready to reenter, nearly every system on Faith 7 had failed, and Cooper was flying by the seat of his spacesuit (which also failed during the flight leaving Cooper too hot and too cold).
There was debate within NASA about a seventh Mercury flight. Astronaut Deke Slayton had a medical problem, so he did not fly a Mercury flight. The next slot in the rotation would be Alan Shepard. They dubbed his flight Mercury Mark II. It was to be a long duration flight in a slightly modified Mercury space capsule. Alas, when Cooper's day long flight was successful, project Mercury came to a close. NASA was ready to move onto Gemini.