I am giving life to player every time they die (through rewarded ads) but after every 5 ads I want to say you can get more lives (or can watch more rewarded ads) after 30 mins or hour by displaying time (through clock or timer).
You need to get the time at the start of the period using something like
DateTime startTime = DateTime.Now;
Use a coroutine or update method on an object controlling your adds to check
TimeSpan ts = DateTime.Now - startTime
You can then check ts.TotalMinutes to see if enough time has elapsed.
Watch out for the user exiting the program and relaunching, resetting the countdown. In that case you need to store the start time to a save file or other means.
To show the countdown on screen, just query the TimeSpan object.