If you are using that stuff, it does not evaporate out. Therefore, if you do a 5 gallon water change, it only needs 1 tbsp of salt to replace what you took out.
There is much debate on the usefullness of aquariums salt here. Some say it is not usefull at all. Some say it can be usefull if you add it when needed - they say it is the change in salt content in the water that helps. Some people swear by it as a general tonic they use all the time, but these people tend to only have anecdotal evidence of fish staying healthy.