Here is the thing. I have said it to many people and have said it on hangouts before as well.
IF you use it when you are roaming data or HACKED your PRL to run off of another carriers tower...
THEN you will stand out and be flagged if you go over. I have seen people get letters for anywhere between 5 - 10 gigs worth of data. Once again this is ONLY when roaming or hacking your PRL.
IF you use this on your regular sprint service AND you are not in roaming, then you should be fine to use as much as you want. I have personally used around 20 gigs a few months. Never recieved a letter. I would say to be safe try and not use more than 10 to 15 gigs a month. Less if you can but really I never had any problems.