QUESTION:
How long does Pathfinder last when compared to the Energizer Bunny? 

ANSWER from David Mittman and Richard Ewell on November 22, 1996:
When we talk about Mars Pathfinder, we really mean three different
spacecraft designed for three different tasks: cruise, lander, and 
rover. From launch to Mars landing on July 4th, 1997 (about seven
months), the Mars Pathfinder spacecraft is in its "cruise" 
configuration. The lander and rover are enclosed in a protective 
shell, and the shell is attached to a "carrier" which guides the 
spacecraft to Mars. This "cruise" spacecraft only lasts seven 
months, after which the "carrier", or cruise stage, drops off. The 
protective shell then enters the Martian atmosphere and allows the 
lander to land safely on the surface.

During the first day on the surface the lander opens up like a flower 
and the rover is then able to drive off to explore the surface. Once on 
the surface, the rover, named Sojourner, should survive at least 
seven Martian days although it may survive as long as a complete 
martian month (30 martian days). The lander should survive at least 
30 martian days but may survive as long as one Earth year.

A Martian day is about 39 minutes longer than an Earth day.

You asked how long the Pathfinder lasts compared to the Energizer 
Bunny. Using the most recent data (1996) available from the 
Energizer company, we estimate that it would take about 22 
batteries to power the Mars Pathfinder spacecraft during its cruise 
to Mars. The problem is that you'd have to change the batteries every 
two hours! So, while the Energizer Bunny may keep going and going, 
Mars Pathfinder needs a power source that is a little more reliable. 
That's why we rely almost exclusively on solar power during cruise. 
(During our cruise to Mars, we dip into our battery only during 
maneuvers that require us to point away from the Sun.)

ANSWER from Guy Beutelschies on May 28, 1997:
Pathfinder is designed to withstand the environment on Mars.  The materials
we used will not become corroded for many years, which will be long after
our mission is over.