Rather than a fixed number, it should be a percentage of maximum attainable volume without FUP. Say 10%.
By maximum attainable volume without FUP, do you mean the volume of data consumed if theoretically one kept maximizing the utilization non stop for 30 days?
If so, assuming that one opts for an 8Mbps connection and gets 1MB/s downstream bandwidth. Then in one day 84.375Gigs and in a month that would amount to 2.472TB of data.
10% of that might be too little for heavy users.
Having Happy Browsing Offer has spoiled my Dad and I who aren't satisfied with anything less than 1080p on Youtube and 1MB/s in download bandwidth while downloading stuff off the internet. FUP makes no sense in this day and age of Steam, Netflix, Amazon Prime, Soundcloud, Hotstar, huge updates to basically everything from app updates to driver updates and not to mention Windows updates.