My all-on-4 bridge is loose. How long can I wait before it causes damage to one or all implants?

If an implant is going to fail, it fails usually within the first year that it is in your jaw. Unless something has dramatically changed with your health, it is not likely to fail "all of a sudden" after 3 years. 
The most likely situation is that some part of your appliance is loose. There are many screws in the All-on-4 treatment and they can loosen over time and cause things to move. You should see your dentist ASAP if any part of your All-on-4 is moving. 
A loose screw means that your prosthesis may be moving around and applying pressure unevenly at different implant sites. One of the most important factors to maintaining healthy implants for a long time is to have a well distributed biting pressure at each implant site. A loose prosthesis does not allow this pressure to be distributed well. 
This is why it is important to go to regular check ups for your All-on-4 prosthesis. By following a good check up schedule, any loose screws or any other little thing that is out of the ordinary can be adjusted before it becomes a problem. 

This is quite common with all on 4. You need to see your surgeon right away to ensure nothing serious happened to your bridge. Your bite could be off, an implant could have failed, a screw joining the bridge and implants could be loose or the bridge itself may have cracked.