It is rude not to send out a thank-you card within a few weeks.
When my husband and I got married, we wrote our thank-you cards out on the plane on our way to our honeymoon. You're just sitting there anyway, why not make the time productive? We didn't mail them until we returned to the US a week later, but even so, they got out in plenty of time.
We attended a wedding last year and only just received the thank-you card...
This was the same wedding where they ran out of food with fully a quarter of the guests unfed.