In most cases nothing in particular happens to an implant for many years — it stays in the bone and does its job as before. It cannot be guaranteed, however, that after 10 or 20 years everything will look exactly the same.
With age, the tissues around implants naturally change. Among other things, the bone may gradually shrink, the gum line may change, and so may the bite. The pace of these processes is individual and depends on many factors, so it is impossible to predict in advance exactly what a particular patient's situation will look like in fifteen years.
That is why regular checks of the implant's condition matter more than a promise that it will "last a lifetime". Check-ups let us monitor the bone level, the condition of the gums and how the crown is working, and react early enough if something worrying begins to happen.
It is also worth remembering that the implant and the crown are two different things. The implant itself can stay in the bone for many years without needing replacement, while the crown may in time need repairing, replacing or refreshing.
That is why during treatment we record the model, size and batch of the implant used in your notes. This information may matter in ten, twenty or more years, should a repair or replacement of parts of the restoration ever be needed.