It is brought back to earth on board either the space shuttle or a Russian Progress spacecraft.

The Progress is a Russian expendable unmanned resupply spacecraft originally designed for resupplying the old Soviet/Russian Mir Space Station. Launched aboard a Russian Soyuz rocket, it currently resupplies the International Space Station about 4 times a year; however, since it docks with the ISS and allows Astronauts on board, it is technically considered a manned spacecraft.

The Progress remains docked to the ISS until shortly before a new one is scheduled to dock with new supplies; waste is then loaded aboard the empty Progress, and then jettisoned from the ISS where it burns up in the atmosphere.

A robotic spacecraft, is a spacecraft with robots in it,

Spacecraft is both singular and plural. You can have one spacecraft or many spacecraft.

254 days (approx.) to get to Mars, but a bit faster coming back to Earth as a spacecraft would be travelling into the gravitational pull of the Sun