Your contact lens case should be changed at least every 2 months.
While your multipurpose contact lens solutions like OptiFree, BioTrue and Revitalens kill bacteria, they don't kill ALL of the bacteria, fungi or other organisms that can flourish in your case. Soaking your contacts overnight in an old case can actually increase the chance of infection if your case is harboring microorganisms.
Don't fret, you don't need to go buy a new case for $5 each. Many companies now include a free case when you purchase a larger bottle of contact lens solution.
And don't forget to change your solutions daily - don't just top it off!