I drink three glasses a day, and the sizes vary between 6 and 14 ounces I guess, it really depends on the fruits I juice and the time I have. I am now starting to understand a bit better how much I can get from each fruit or veggie, so it should stabilize a around my original plan, which was around 10 ounces.
In the morning and at noon, I normally have a fruit juice, and in the evening, a veggie juice. But I find it hard to stick to this, as veggie juice are very mild and lack flavor.