There is no hard and fast answer. Every patients anatomy is different and how much to fill the lip is a personal preference. Some like their lips very full and thus require a larger amount of filler ( 1cc or greater). For someone who wants a subtle enhancement, there is always the option to inject only along the borders to create more structure to the lip and show more of the red lip. This only requires less than 0.5cc per lip. Another option is to only add to the central lip itself to create volume and a nice pout or volume to the middle part of the lip.
You need to consider, also, that there is a balance that must be achieved between upper and lower lip so that a natural look is maintained. so don't ignore the lower lip if it needs some volume to maintain balance.
Consult your dermatologist or other experienced physician and go over these options.
