A game that has a lot of content is xcom enemy unknown. I looked on the internet and people estimate that it has roughly 20-25 hours of gameplay, depending on what you do, and that's just the first playthrough. Some videogames i played have the same amount of hours of gameplay or less.
As a personal note 25hr. is not what I would regard as "a lot of content" but that's mostly besides the point as it's not the average length of offerings but the overall range of offerings which matters, something iOS is doing mostly well on.
The problems with iOS as a serious gaming platform, at least for actual enthusiasts, isn't the titles on offer but the marketplace itself. Apple is the problem here.