The retention shouldn't be limited for that log. On my lab AMP, I have entries that date back to the installation date.
Try grabbing the diagnostics tarball from the System -> Status page (the diagnostic tarball link is near the top of the page). When you extract it, there should be a var.log.system.amp_events which is the log that displays on System -> Event Log. If it's showing more data than you saw from the UI, it's possibly a browser caching issue. If the log is really only showing 14hrs, then a support case should be opened to investigate.
In the latter case, you could try a few things before opening a support case:
1) try restarting the AMP services (this restarts the logging processes)
2) check disk space