It really depends upon what you are doing. Stirring is a very complex thing. Prolonged stirring can alter the crystal morphology of a compound. To violent stirring can chop up crystals so that you cannot filter them, By choosing the wrong stirrer your mixing may not be good enough, etc.
In your case, if your reaction starting materials and products are stable extending the stirring time should not matter too much. I have experienced epimerisation by stirring for too long, but I was lazy and left the reaction while I had lunch, I should have managed my time better.