Reverify but don't disturb the customer if he has no internet connection (endless grace period sort of)Answered

It's been recommended that the customer is reverified occasionally. I want to implement this but using a different approach. I don't want my customers to know that I am reverifying them.  I just want to do it quietly behind the scene. If at any time I discover that the product key is not genuine, I simply deactivate the customer and ask them to buy a product key. 

 I just want to simply check occasionally but if the customer is not connected to the internet, I don't want to disturb them with a reverification request. So how can I possibly implement this? Would making the grace period longer than the  days between checks help in achieving this? 

Thanks.