How long should spokes last?
This largely depends on how frequently you check the true and tension of the spokes.  If one gets loose you are likely to break spokes frequently, and once you've had a few break the others are likely to go more frequently.

As part of my daily pre-ride check I spin the wheels looking for significant wobbles and will take care of them right then.

As part of my monthly check (usually the first weekend in the month) I squeeze pairs of spokes together around the wheel to check that the tension is approximately the same.  If spokes are loose or if the wheel is out of true I remove it and use a truing stand to true it up.

My general rule of thumb has been if I've broken 4+ spokes it's time to have the wheel rebuilt, which at my LBS cost me about $60-80 depending on the quality and quantity of spokes I choose.

Breaking spokes is not usually a reason to need to buy a new wheel unless there is some kind of defect in the rim or the hub.