Right, I was confused by your initial post, as MB and Mbps are two very different measurements.
You also seem to have a slightly unusual concept with regards how to how your connection is being utilised.
Even if you can see in real time how many Mbps you're using at a specific point in time, it's not as if that's what's being utilised at all times.
In fact, most of the time, our internet connections are idle, since unless you're proactively sending or downloading some data nothing is being used.
As such, consider what you're using your connection for. Even if you're streaming a lot of videos, 120Mbps is more than enough, as a single 4K video stream from Netflix as an example, is only using up a maximum of 25Mbps, so you could stream four 4K video streams and still have bandwidth over on your current connection. Currently no streaming service have bandwidth requirements that exceed 25Mbps per 4K video stream.
However, say you download a lot of large files, say 100GB or more a day, then you might want to consider a faster internet connection, as that would, assuming where you download the data from is fast enough, cut the download time by a third or more.
Likewise, if you upload a lot of data, the same is true.
It's nigh on impossible to get an accurate reading on how much of an internet connection is being used over a period of time, as you can only get snapshots of a given time.