Max Number of Cc's when Adding More Saline to Implants?

Usually there is a a small amount of overfill that is allowed by the manufacturers, about 30cc in most cases. This makes little difference if the breasts are very different in appearance.
Exceeding this amount can result in deflation or rupture of the implant, or in a deformation of the appearance of the implant since it is made for a specific volume, and not to be excessively overfilled.
Dr. Canales has pointed out that implant deflation and weight changes can change breast appearance, as can capsular contracture, drooping or loss of breast tissue over time.
If you want to attain the very best result, you may want to have your situation reevaluated again by an experienced plastic surgeon rather than to try to add more saline to an 8-year old implant.

If you have implants that are 8 years old, they already have some structural fatigue. They may rupture while trying to add fluid, or shortly thereafter. It does cost money to buy new implants, but compared to the overall cost for surgery it is not that much, and you are ensuing the risks of surgery to even consider adding volume. To risk needing another operation in a short time is not worth the possible cost savings from recycling your implants.

It is not a good idea to add volume to existing implants. The best option would be to exchange your current implants for new implants. You may also want to consider changing your saline implants to silicone implants. In either case, make sure you have a good discussion with your surgeon before embarking on yet another breast surgery. Good luck.