97 maxima Check engine light on

[fastmaxima]

Hello all--My service engine light came on--i have a scan tool and it shows P0325 and P0136--The rear o2 sensor--My question is--Why when i reset the code light off--I can drive almost 250 miles before the light comes back on??--I would also like to know how hard it is to change that rear Bank 1 sensor 2? Thanks all and have a great day!!!!!