It depends on where I am and what I do and what cup I use.
When I visited NYC i bought a cup which I prefer to use at the moment but it is more than twice as big as my other cups (20oz)
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The others have about 6,7oz.
So when I use the bigger cup I only drink one and when I use the smaller ones I drink three or four.

AFAIK 15-20oz are considered normal.

Kathryn