Let's see if TIHWD will peak next week on Billboard HOT 100, if peaks, ok. If it start falls, it's time to release a new single.
I know it's basically impossible BUT:
she should release WOA next month to Radios play her songs again (I saw they are testing it on radios, right?), and then perform this song in TV Shows or whatever (She has half of october without concerts). I know it'll tire her, but in November she should try to fly from australia to perform at AMAs in U.S!!
Then in January/February she releases Legendary Lovers, her name will be higher again because of WOA, so I think at this point LL would be a "hit", not a smash, but a memorable single. And if she performs an acoustic version @ The Grammys, it would be a perfect way to finish PRISM era.
Both songs are super hits, but if released in the wrong order and time...will fail.