Pulling up the man leave page told me the very simple usage. Simply type:
% leave +hhmmwhich will set an alarm for that many hours and minutes from the current time. To use a fixed time, drop the + flag, and it will allow you to set the leave alarm to some time in the future. You are reminded at five and one minute intervals. Nice thing about it in OS X is it actually audibly beeps at you.
A nice augment to leave is calendar, which is a little more flexible, see the man calendar page for more details.
[robg adds: Speaking of which, it's time for me to leave! Have a great day, everyone!]

