The sign should read, "Guests' rubbish only" (the rubbish of the guests).
The sign should read, "Guests' rubbish only" (the rubbish of the guests).
The sign should read, "Guests' rubbish only" (the rubbish of the guests).
The possessive form of the plural noun guests is guests'.
example: Our guests' comfort is our first priority.
If it means that only guests are allowed to deposit rubbish, then put an apostrophe at the end of guests: "Guests' rubbish only."
There are numerous factors to consider, time of day, duration. etc. No less then 12 ounces per person, however you know your guests best.