It would depend on the internal volume of the port and that can be variable. Also you would need to take into consideration any priming volume in your access device.
Our huber needles with extension have a priming volume of less than .3 ml and usually 1 ml is enough to clear the port.
I would recommend checking your manufacturer's product info for priming volume and go from there. In adults, 2 ml is probably the smallest appropriate volume but those who work with adults can give you more specifics.