H1 to F1 question

Can anyone help to give your opinions for the following questions? THX:
1. What's the legal time gap between H1 and F1? i.e., after laid-off of H1, how soon the application of F1 should be filed?
2. can anyone provide a sample of explanation letter, which helped to change H1 to F1 successfully?
3. Do I need to show I don't have immigration intention? As you know, H1 allows us to do so, however F1 doesn't.