It helps to be good, and it's even better to be lucky and good; they were extraordinarily fortunate to end up in that little crater, where they could get some nearly-immediate scientific results. Had they landed on the plain in between, they probably would have ignored the little crater and headed straight for the big one; and though it looks promising, that might not be a better scientific 'target'.
I just read an article that indicates they think they can get around a 200-sol lifetime out of each lander, so they've still got some time (Spirit is on sol 78, Opportunity is on sol 57).
How long until the Earth and Mars are on opposite sides of the sun? We will lose contact before that. It takes Earth 182 days to get from one side to directly opposite on the other. The question is by how much does Mars lag behind?