The timeline for thanking someone depends, in part, on the situation that prompts this correspondence. With a job interview, for example, sooner is always better. For a baby or bridal shower, however, you may have a bit more leeway. If possible, aim to complete and send these notes within two or three weeks of the occasion. A casual thank you offered to a friend or family member can be sent at your discretion — but if it's in response to a specific action (such as taking out the trash), it should be given soon enough for the recipient to remember the situation you're referencing.