Thank you cards

Growing up, I don't recall sending thank you notes out when I received birthday gifts.
So it's not really ingrained in me to send them out on behalf of the girls when they receive presents.
Of course, we'd receive them from S's friends, which made me think we need to polite as well.
But, well, while my intentions were good, I don't think I've sent thank you cards out in the last three years. Maybe I did once. Or maybe I meant to but only got as far as filling them out.
This year, I found packaged invitations that came with thank you cards. Super! Now I will have to send them out!
Well, here we are, two weeks after S's birthday party and the cards sit on the end table. I guess the reason mostly is because we've been battling illness.
But I know too I just plain forgot about them. Didn't help that they were under a pile until tonight.
So, for those of you who attended S's party, I apologize for not sending out the thank you cards. I hope to get them in the mail...soon. I wonder what the etiquette is for how long you have to send thank yous out.