they last ages, well mine did anyway, just like any other car really when it cracks replace it only worth around $25 genuine anyway, there are worse things to worry about like tyres.!
Never had a serpentine belt break on me, they're very durable. However if you are having regular breaks I'd take a look at how well your accessories and tensioner are mounted, if you have one of them out of alignment at all it may affect belt life.
i bought a gates belt 4 yrs ago when i added s/c and it was fine but started to slip a little bit so i went and got new one but silly me bought a repco one 40 BUX and was a little bit bigger and slipped a little bit so i sprayed it with belt grip drove home turned car off in the morning started it drove 1 k and it shedded and broke off
stupid repco said their brand was the same but ill buy a new gates one in future and pay more
mine broke once because the tension pulley was a bit c*ck eyed and it stripped the belt. change the pulley and no prob for the last 2-3 years. i always carry one in the boot anyway.