6-8 weeks in Phineas and Ferb?

I was watching Phineas and Ferb S02. In the episode "Phineas and Ferb Busters" , I was surprised to find a dialogue.

At the Doofenshmirtz Evil Inc., Heinz Doofenshmirtz gets a package in a mail. Norm receives it and says,

You will be delighted if you get a package in 6-8 weeks!

I was startled after hearing the words 6-8 weeks because it is Stack Exchange's infamous meme originated by Jeff Atwood. But its earliest reference was from Muppet Babies. Crazy Frog also has a reference of 6-8 weeks.

Now, I am interested to know from where the animators got the idea of using it because the season 2 was from 2010 which is after the starting of Stack Overflow and it became famous by then. Is it a coincidence that they might have been used Jeff's words?

From where did the animators get the idea to refer 6-8 weeks? Is there any word from the animators?