Craft Redis Cache - Get Time Left?
Craft is piggybacking off of Yii's caching functionality for 99% of this and their cache classes don't expose any type of TTL functionality.  My guess is that because of all the different caching providers they support (memcache, xCache, WinCache, ApcCache, etc.), it looks like Redis is the only one that has this sort of functionality.

There isn't a clean way for us to add this to Craft itself, so I think a proper fix would be to add it to Yii, but considering Yii 1 is in bug fix more and not getting any more enhancements and Yii 2 has dropped support for Redis, that seems unlikely.