Curious as to your intent here. It would seem one would use this or size, but I'm not sure its useful when combining. A time-to-live helps protect against stale data, which might be combined with automatic refreshing to avoid exposing a latency penalty for hot entries. A time-to-idle is a poor man's hint of recency/frequency to the eviction policy. It was free to implement and a common feature, but less obvious when useful. 