Hermione failed the test and nothing else has happened. From what I know, she should have asked for a favor by now but she hasn't. I'm on day 28. 

Similar problem.  I've gone past 100 days, heavily emphasizing hanging with Snape, had the scene where tutoring wasn't possible, and since then no favor discussion.  

You have to read the fiction books

Read all of them.  After all, had to do something with the time while waiting.  

I am maybe to late, but for progress in this point you need -chit chat- whith Snape during daytime, he says about tutoring and next few day Hermione is come