You need to replace the solution every thirty days. Each pair of contact lenses has an expiration date, usually packaged to last for about two years. If you don't wear them too often and take proper care of them (wash them and replace the solution every thirty days if you don't wear them) then they should last for about two years. Does that help?
@kkoganei1328: A lot of people don't realize that they need to see an optometrist, so this thread exists as a general first reference for people who have never talked to their docs about contacts. This by NO means replaces a conversation with the doctor, but gives the users a launching point for contact use as well as along-the-way tips and pointers for cosplay specific contact users. At least, that's the way I feel about it