Let's take a closer look at the nuclear cell.

According to According to NASA;

Radioisotope power systems are generators that produce electricity from the natural decay of plutonium-238, which is a non-weapons-grade form of that radioisotope used in power systems for NASA spacecraft. Heat given off by the natural decay of this isotope is converted into electricity, providing constant power during all seasons and through the day and night.

It contains about 10lbs of plutonium-238. Great! Now how long will that last?

Again, from the same NASA page:

This type of power supply will give the mission an operating lifespan on Mars' surface of a full Martian year (687 Earth days, a little less than two Earth years) over a wide latitude range.

So we have about 2 years.

Let's take a closer look at the nuclear cell.

According to According to NASA;

Radioisotope power systems are generators that produce electricity from the natural decay of plutonium-238, which is a non-weapons-grade form of that radioisotope used in power systems for NASA spacecraft. Heat given off by the natural decay of this isotope is converted into electricity, providing constant power during all seasons and through the day and night.

It contains about 10lbs of plutonium-238. Great! Now how long will that last?

Again, from the same NASA page:

This type of power supply will give the mission an operating lifespan on Mars' surface of a full Martian year (687 Earth days, a little less than two Earth years) over a wide latitude range.

So we have about 2 years of the rover being able to be run on nuclear power. After that, though, it should be able to run for another twelve-ish years on its MMRTG.