As I saw the challenges that used did not require you to restart your file, it usually meant like... People that have completed files and use them as their "main" game, so to speak. As in, that's where you're done, you beat the Champion, you trade or battle, etc. So without having to delete that, you're able to pick up a new team from scratch and do this challenge. However, if it wasn't an option before, I think those could also benefit from adding the option to use a new file. I personally enjoy a fresh playthrough with that team and getting the full experience with them rather than bum rushing to raise them all post game, however I won't say that challenges like that are a bust. Good example of that would be Stadium Arcadium a few years ago.
But anyway, having a time frame isn't bad, it's just that having it too long makes the event feel less...eventful? Aha. When it's running for two or three months, it doesn't feel like anything you have to hurry and jump on while it's still available. When it's up for a month, that's usually plenty of time to finish a Pokemon game (and people know their abilities best; if you play fast, you can join late and still finish and if you play slow, you can start at the beginning and still pace yourself for the most part). I always ran mine a month at max because I felt like that was plenty of time, but also not too long of time. Usually the challenges were more centered towards holidays as well, so obviously you don't want like...a St. Patrick's Day challenge running into May. The novelty has worn off by then.
But anyway, having a time frame isn't bad, it's just that having it too long makes the event feel less...eventful? Aha. When it's running for two or three months, it doesn't feel like anything you have to hurry and jump on while it's still available. When it's up for a month, that's usually plenty of time to finish a Pokemon game (and people know their abilities best; if you play fast, you can join late and still finish and if you play slow, you can start at the beginning and still pace yourself for the most part). I always ran mine a month at max because I felt like that was plenty of time, but also not too long of time. Usually the challenges were more centered towards holidays as well, so obviously you don't want like...a St. Patrick's Day challenge running into May. The novelty has worn off by then.