When should the metal clips be replaced on a brompton folder?
The Brompton owner's manual states:


  SECTION K: ROUTINE REPLACEMENTS.
  Planned replacement is advised to ensure both safety and good performance. The suggested intervals between replacements are for bicycles subjected to normal use: the most appropriate timing depends on the conditions of use and your riding style.
  
  
  ALUMINIUM COMPONENTS: as on other lightweight machines, aluminium alloy is used in
  the construction of the Brompton, and this material has a finite life before failure. In normal use, the risk of aluminium fatigue failure is remote, even after many thousands of miles. However, the risk of failure increases with use, especially with hard riding or other severe loading: as such a failure could cause injury, the hinge clamp plates, handlebar and chainset should for safety be replaced every 5,000 miles (more frequently on any machine subjected to hard use), and we recommend that these items are anyway checked regularly.
  


As the manual itself notes, that is an extremely conservative number, but considering the hinge plate itself is a £4.00 part it seems better to err on the side of caution.