I think subscription-based apps have always been able to offer a free trial period, but you cannot require a one-time paid upgrade to the full version after X days.
If you happen to know of any sources for that policy or any examples, I'd be interested in seeing them.
It's possible to offer a free trial as part of an auto-renewable subscription,[1] but that scenario is different to the app ceasing to function X days after you've downloaded it.
[1] https://developer.apple.com/library/ios/documentation/Langua...