ALWAYS replace the sprockets if fitting a new chain, it's a false economy not to. If you want the maximum mileage from a chain, then apart from good maintenance, you should buy 2 front sprockets & one rear (steel) with a new chain. Run about half your expected mileage on the first front sprocket, then change to the second one. The reason for this is that the rear sprocket is usually about 2.5 times the tooth count of the front one, thus the chain is always in contact with more rear sprocket teeth than the front & sees a chain to tooth interface 2.5 times less often than the front. It's the chain contacting & fitting into place that wears the teeth & chain rollers. So two front sprockets evens out the wear rate & stops a badly worn front sprocket accelerating the wear on the chain when it makes its tight turn around the front sprocket. Also remeber a slightly loose chain will do less damage to itself & the sprockets than a slightly to tight one, but a correct adjusted chain will wear the least. As to expected mileage, how you use the bike & the speeds you ride at will have a marked impact on the longevity of the components.