It depends on your plan - are you on the newer Verizon plan? Are you watching through the NFL Mobile app or Go90? When you first log in to NFL Mobile it should confirm your plan/Verizon account.
The actual game viewing is free, but data is charged for the commercials.
You may be credited for data used for the ads, but not sure exactly how that part works? I think you have to watch most if not all of that particular game for the credits to be applied.
My husband and I both enjoy watching games via the NFL Mobile app. Here is what we have discovered through personal experience:
The first time we opened the NFL app on our new Pixel phones, the app "confirmed" we were Verizon customers with a qualifying plan in order to get the free streaming of the games. We had to have wifi off and cellular data enabled to do this as the phone had to communicate via cellular data to the Verizon network.
The first month this season (September) when the new "bonus GB" of data was implemented to cover the data use during commercials, I did not receive the bonus data. (This was on my previous phone, hubby did not have a smartphone for the NFL app at this point). I called about it and was given a $15 credit to purchase a data boost, effectively a bonus GB. Sweet!
The next month (or when my billing cycle rolled over, the 22nd of September) I watched a quarter of a game on cellular, including commercials. (Normally we watch over wifi, at home on wifi is normal). Within a day I had a bonus GB added to my data, no need to call in or request the credit or data. And the cellular data I had used for that one quarter showed on my phone as cellular, NFL mobile, but did NOT factor into my plan usage.
On 11-11-16, we upgraded to Pixel phones. I was curious if we would get 2 Bonus GB if hubby and I both used the app on our own line, or if the bonus GB was per account. It took about 24 hours to show up, but after watching a quarter or so (we didn't need to watch a whole game) on cellular on each phone, we had 2 Bonus GB added. That's nice!
Normally we are on wifi when watching the games, but it's nice to know we can watch, data free, AND get 2 GB bonus data to boot. And in our experience, a quarter seemed to trigger the bonus; we've never watched an entire game (or even a half!) on cellular.