Mentor smooth round high gel (400cc) implants - what is the lifetime of these implants?

Generally speaking, breast implants do not need to be exchanged after a set period of time;  if the implants are in "good shape" and patients are pleased, there is no "standard exchange time".   If you have concerns regarding your breast implants, best to follow up with your plastic surgeon.  The best study to evaluate the integrity of the breast implants would be a breast MRI.
 
  Best wishes.

The longevity of implants is unknown. A MRI x-ray would determine if a gel implant is ruptured.Dr.Ted Eisenberg, Board Certified Plastic Surgeon and Author

in that you should never experience a loss of volume but implants can rupture and if this happens, the knee jerk reaction is to switch them out for new ones.  But gel is cohesive now so it all sticks together and should not disseminate into the surrounding breast tissue if the shell fails (implant ruptures).  The government recommends a MRI every 2-3 years to check for the integrity of the implant.  I tell my patients they can always come in for an exam (as there are things that do change if you do rupture) should they have any concerns or certainly before the 10 yr anniversary from the procedure so you can take advantage of the warranty before it changes at 10 years.  If you are having changes in the appearances of your breasts, you should see your surgeon to figure out just what is going on.