Depends where your company is based.

In the U.K I believe you have to store all information for at least 7 years then maybe forced to erase information (as part of the Data Protection Act)

In the Eastern Europe as far as I know ,for example, they don't keep the customers' information for too long

Just keep their contacts so that u will be having their e-mails. And if you want give any special offers anytime you can mail that offers directly to them and who knows...? you may get sign-ups. !!