myversa

http://www.seekingsuccess.com/articles/art151.php3

Think about how much notice you should give. On average, people give 2-4 weeks depending on their level of responsibility.

It's not enforced and you just have to give reasonable time.  It's better to leave until your manager can find someone to replace you.

myversa

I think the employer can fire you on spot if you give him the letter.  Since you give him the letter, you already determine to leave the company already and if the employer does not care if anyone can fill your shoes in the company, he can let you go right away.

In the end, it all depends on your contract.