This varies from different online stores. For most of them, it is decided by the reason why you want to return your prescription glasses. If the prescription is processed wrong( not exact the same as the prescription you entered when placing your order), you get the wrong frames or the products are arrived damaged, then you may get your money back. For other condition, you may get part of your money back or all should be your responsibility.
That will depend on the condition why you want to return your prescription eyeglasses. What's more, different stores have different rules on return and refund. I am sure most stores will ask you to pay for the shipping cost if you return your glasses that you buy online.
Some online optical stores have the policy that some types of glasses cannot be returned, so before you buy glasses online, you should ask them whether you can return them if you aren't satisfied with them.