Lately I have been receive a TON of questions in regards to Wedding-gift thank-you notes.
So here is the answers:
When to mail the note: You have some leeway on timing since your guest will understand that you have quite a few to write, but it is best to get them out within a month after the honeymoon. Although some etiquette professionals say that you have up to three months I would be safe and start writing them as soon as you can.
(sorced from: The Art of Thank You)