Like Sapper said it really depends. Alternators are generally pretty simple. Usually it is hard to physically install them wrong so it could be that it was simply miswired or the belt not properly tightened. That should be about a 10 minute fix. If it is physically on there wrong somehow, that might be a 1/2 hour job, unless it is really obnoxious to get to.
In my experience labor charges will be $60-$100/hr. You might be able to find a shop that will only charge you a 1/2 hr rate. If you want a cheaper option, you could try checking local auto part stores. Sometimes you can find guys that will do work for you out in the parking lot. Or if you have a day or two to wait, try Craigslist. Put an ad for installing an alternator and offer 20 bucks.