In our research, most students do not complete each case more than 3 times. As the online version is most susceptible to abuse with sharing of logins/passwords, we have limited the 12 month version to 3 attempts per case.
We do monitor for sharing of logins, but this tends to keep the abuse down. For those who feel they require more practice for each case, we recommend the 6 month version.
Please note that there is no case attempt limit for the 1, 3, 6 month subscriptions (meaning you can attempt each case as many times as you wish).