Check out Rogue Adventure.
It's a deck building strategy game.
Doesn't get stale fast, since you can use a lot of different strategies, and cards/features unlock over time.
Also you can play as much as you want, unlike many mobile games which use some sort of energy/mana thing.
Only problem is the ad's, but that's a problem fixed by turning internet off while you play. You dont miss out on much, the only thing that uses online is a tournament thingy.
I've probably sunk about 30 hours in it by now, it's very cool. And i've still got stuff to unlock so it's atleast 30 more to come.