How long will a used contact lens still function if kept in a solution?

It depends. If kept in clean contact lens solution that is changed daily, used contact lenses should last a while. At least till the printed expiration date.
Guidelines. Generally, soft contact lenses in approved contact lens solutions can be stored in a clean unopened contact lens case for up to 30 days. However, i always instruct my patients to rinse their contact lenses with fresh solution before placing them in their eyes as a precaution. Good luck and see your eye doctor regularly!