That business about a year is a convenient excuse for procrastination, and I'm not really sure where it started. In fact if you look at Emily Post's book "Etiquette", she states that there's no excuse for a wedding thank you note to be sent later than three months after the event.
In fact, all thank you notes should be sent as timely as possible, preferably as soon after the gift is received as possible. Although it may be a tedious task, it won't become any easier with time. All that procrastination will do is allow other priorities to bump aside this task, until it feels almost too embarrassing to send out a note.
And although Emily Post gives three months as the outer limit for wedding thank you notes, thank you notes for all other gift occasions should really be sent out as soon as possible, preferably within two weeks. A prompt note shows the giver just how much you appreciate the expense and thought that went into the gift. If you need some help finding the right words to say to start your thank you notes, visit The Do's and Don'ts of Thank You Notes.