FW letter also means loss of perks like no profit share, (like it mattered the last few years lol) and in some cases they may restrict or remove staff travel, it depends on the position of the employee and in some cases their manager but in general it means one more fcukup or you piss off the wrong person and you are donezo.
IIRC minimum one year but I seem to recall some had them for two. Suppose it depends on what the warning was for.