I've no idea of the best practice for clearing the app cache. I always used to assume once a week, but it's been a long time since I remembered to do that. I'm guessing that you can probably leave it until you encounter a problem which may be caused by the data files becoming corrupt, and then clear the cache.
Bear in mind that every time you clear the cache, the phone will have to rebuild it, so I'd say that once or twice a day is far too frequent. But that's just my view.