Google does not provide us any way to manually set the expiration limits of the verification cookies. This is an excellent case to offer up to them as a suggestion for them to add and option during issue time to auto revoke the code after a certain amount of time.
Until Google implements some form of expiration control you could as manually change the cookies and set their expiration date to 30 days out from the day you change it. A Google search for *"cookie editor" will give you many results for plugins for Chrome, Firefox, Opera, Safari. You should also be able to narrow down results for an IE plugin, regular program, or even a standalone or portable application as well. Editing cookies is something that has been around for plenty long enough for developers to have many different solutions available, so you should be able to find something that will fit your particular needs and situation.
It is also ill advised from a security point of view to be providing a blanket authorization on systems that may be open to public use. Cookies are normally user specific so you may be alright if you use separate secured accounts to log into these machines.