Where to find expiring data for Playstation Plus on the PS4

Easiest way is to go to the Sony PSN website and look under Media (hopefully you've downloaded a few of the Instant Game Collection). You'll see the expiration date which correlates to your PS+ expiry date.

It's in the settings menu, with your other Account information (I forget if this is it's own top level thing of if it's under Play Station Network) there's a "Services" section. Inside Services you'll see Playstation Plus and you can see the begining/end of your service date and your auto-renew setting.

Log into SonyEntertainmentNetwork.com and then go to this address:

https://account.sonyentertainmentnetwork.com/cam/media/media-list.action

You'll see all your purchased games along with the free PSN games. The latter ones will all have the same expiration date.

That date is your Playstation Plus expiration date.

Disclaimer: This answer is correct as of this writing but might become incorrect if Sony changes their website.