Bike spokes break most commonly due to wear and tear. A high-frequent cause for spoke breaks is that the rider has hit a curb or pothole, doesn't maintain the bike well, or the passenger is too heavy for that model. Rougher terrain will also deteriorate the rims faster, which in turn deteriorates the spokes faster.

How long should spokes last?

In my experience, spokes last virtually forever. And I've got 30lbs on you. Of course there's too many variables to say "how long do spokes last". A low spoke count poorly built wheel with a heavy rider is going to go through spokes much quicker than a high spoke count quality built wheel with a regular sized rider.

How often should you check spokes?

You can do a quick check by plucking the spokes like a string. They should all have a nice high note. You should have them trued and tensioned about once a year if you ride often. To be honest I don't check every ride, especially since I got disc brakes.