How long should spokes last? Trek Fx 7.3, 1 year old, 1900 miles

3 days short of my 1 year bike purchase anniversary I discovered some broken spokes on the rear wheel. The bike shop gave me a new wheel they had in the shop, an extra wheel they had and said it was better than my wheel. There was no charge for this I presume because it was under a year and though I'm not sure I would guess Treks warrenty is 1 year. 2 weeks later another spoke breaks and I replace that at a 30 dollar cost. The "sales" person tried to sells me a $160 wheel saying that factory wheels are junk and this expensive one is so much better.

This morning which is about 3 weeks from the last spoke, another spoke breaks. I commute to work and this is becoming a problem. I probably need a new wheel but I'm debating whether I want to bother with this shop anymore. 

Any thoughts? 
thanks
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.