Is It Normal to Have Some Fluid Build Up (Shown on MRI) Behind Silicone Implant After 7 Years?

I have had a Mentor implant for 7 years. I have an MRI done annually to look for cancer on that breast. Other breast was removed due to cancer. Each year the last 3 years, the report shows fluid build up behind the implant. My PS said to "keep an eye on it". So how much fluid can build up before it's a concern? And what could or should be done about it? Is it anything serious? I have no symptoms.