If you do not use a revolving credit product for a given period of time, the issuer might close it.
What is "a given period of time"?
There is no definitive answer to that.
A year is an interval that some issuers might use.
I would suggest at a minimum of every 6 months putting at least a token charge on cards that you wish to keep.
Issuers of credit do not want their products to be idle only to then be used "in case of emergency break glass" scenarios.