My previous phone was a iPhone 6 with 16 GB and the last 3 - 4 months was a struggle.
Now I have a iPhone 7 128 GB, a year in and it's currently sitting on 48 GB used, I have quite a lot of music on it just because I can.
I guess I'll come right with 32 GB if I have to, but 64 minimum will be a safer bet.
At this stage I don't really see the need for 128 GB even though I have it, and I definatly don't need more. Keep in mind I don't really do photos and videos, which seams to be the things taking up more and more space on devices these days.