It is hard to say for sure, as data consumption depends on various factors including the lighting condition, the amount of movement detected, as well as the number of Camera Units you own.
However, we can tell you that Alfred does not stream video continuously and we pride ourselves in our image compression technology. On the Viewer end, data is used when you watch live feed, play a Motion Detection video, and receive a push notification.
Live feed could consume anywhere between 20 to 70 KB per second depending on the lighting condition. A Motion Detection snapshot is around 20-50 KB, and a 5-second Motion Detection Event is about 50-80 KB. If there is continuous movement, the Premium version could need 2.5 times to 3 times as much data for streaming. If there is no movement, then the difference isn’t big. The same goes to the live feed with Low-Light Filter on.
With that said, the best way to get a more accurate idea about how much data your Alfred needs is testing it out for a day or two. We often get this question before Superheroes go on holiday. Why not let Alfred run nonstop for a day or two and figure it out before you jet off?