Hand Limits limit the number of cards each player can have in their hand.

Whenever a new Hand Limit is played, place it face-up anywhere on the table. If there was already a Hand Limit in play, discard it and replace it with this new one.

It's rules change takes effect immediately, so if it is currently not your turn, you have to adhere to the Hand Limit. In other words, players would have to discard cards to comply to the new limit you just played.

For example: If you played a Hand Limit 0 card, all other players would immediately discard all their cards in hand. Then at the end of your turn, you would discard all the cards in your hand.