When shopping for a chair mat, you'll want to consider the thickness. A good rule of thumb is that the thicker the mat, the longer it will last. For example, a 3/8-inch thick mat will last about twice as long as a 1/4-inch thick mat. Of course, the tradeoff is that thicker mats can be more expensive.
If you're looking for a mat to use on carpeting, you'll want something on the thinner side so it doesn't get stuck in doorways or under furniture. For hardwood or tile floors, a thicker mat will provide better protection against wear and tear. Ultimately, it's up to you to decide what thickness is right for your needs and budget.