Theory to Backup 4-8 Week Wait for Follow Up Survey?

Hi everyone, on the Resources page, it says to wait 4-8 weeks between Immediate and Follow-Up Surveys.
Anyone have a source to point to that backs this claim up? I tried emailing Project Outcome but no response yet. It seems to be based on their experience. How long do you wait before sending follow-up surveys?

The link to the 4-8 claim is

Hiya. I don’t know about the theory but I can speak from experience. I started with 8 weeks before the follow-up surveys and got very little response. One of my surveys got 0 responses after a week!

For programs in January, we are going to start sending follow-ups after only 4 weeks. While I want to know how people used what they learned, I want to ask while they still remember that they learned that from our class. I’m hopeful that our response rate will go up.

I have sometimes used the follow-up surveys as the only survey – I have found the questions sometimes work better than the immediate survey questions depending on the program.

However you will not be able to compare data.