How often do I have to switch my contact solution from my contact lens case?

Clean the case. Contact lens cases are commonly neglected in those who otherwise care excellently for their contact lenses. The case should be cleaned thoroughly, using soap and water and allowed to dry completely at least once per week. Some recommend boiling the case or ethyl alcohol but this must be thoroughly removed before the case is used. Some replace each 3 months.
Every use. Don't re-use the solution in the case that has already cleaned the contacts once. Don't add more without first eliminating the old (known risk for infection). Follow your eye care provider instructions.