pro sites - lifetime subscription

Hi, thanks for taking a sec.
I'm looking to add prosites to my mu install.
I'd like to be able to create a coupon that gives lifetime access.
Whats the best way to achieve this.
Or do I need to renew them each year?
If so, will there content still be live once they re-add the coupon?
I'd like to keep it as simple as possible and also be sure that their content/sites will not be deleted or lose any of the content they've posted.
Thanks so much for your help & patience,
Yohan