When to Send a Thank-You Note

Event Essential or can I decide for Myself?
Wedding Gifts Obligatory, within three months–even if verbal thanks have been given.
Mailed gifts Yes, this is mandatory within two to three days from when gifts are received. Even if the gift received was a thank-you note, the sender wants to know it arrived safely. Give them peace of mind.
Birthdays, Christmas, and other gift-giving occasions Easy one. A note should be mailed within two to three days from when the gift is received.
Wedding or baby shower As in other gift situations, a note is in good taste and shows appreciation to the giver.
A dinner party A note is always a good idea, and will probably guarantee you an invitation again sometime.
Staying with friends A note is important. Close friends or family may be happy with a phone call of thanks, but they really deserve better. Send a card!
Gifts or help during an illness When you are feeling up to it, a note is a must.
Receipt of notes or flowers of condolences Always send one. Always. Someone was concerned for you, express your appreciation.
Business gifts or Entertainment Never a must, but you do want to make that sale, right? You want to bet on the winner? A note can only help you foster a good relationship with clients, fellow employees, or your boss.

The above chart is from Writing Thank-You Notes, Finding the Perfect Words by Gabrielle Goodwin & David Macfarlane.