The ProxySG appliance caches CIFS objects for as long as the object is valid. It queries the object's properties (such as size, create time, last access time, last modified time) everytime the CIFS Proxy intercepts a client request. If file properties on server matches the cached object's properties, the client's request are served from cache.
There is no set period of time how long the ProxySG appliance keeps a file in cache. The CIFS object cache is stored in the same way other service objects caching are. All ProxySG appliance models have limits on their disk space for object caching (Management Console > System > Resource > Disk Use). If object caching reaches a point where all disk space is occupied, the oldest objects are discarded.