If you know that in batch mode, a well agitated quantity of salt at the appropriate concentration requires 2 minutes to dissolve completely, then you should allow for a residence time at least five times that long, 10 minutes, for continuous operation.  If the exit concentration must be completely uniform with no remaining salt crystals, then a residence time ten times the batch time is probably best.
 
Under continuous operation, the water and salt may be brought into the top of the stirred tank, but the salt solution should be removed from the side of the tank, either as overflow (at the side opposite the feed) or at a controlled rate below the surface.  The mixing intensity should be designed to keep all of the salt crystals suspended off the bottom.  However, removal at the bottom will always risk some undissolved salt crystals being carried in the exit stream.

The answers by this expert are based on the best available interpretation of the information provided.  The consequences of the application of this information are the responsibility of the user.  If clarification is needed, please submit a further question.