When To Use Implicit wait and where to Use

I am having some confusion regarding Implicit wait method provided by Selenium Webdriver.

1. When to Use Implicit wait

   a- For Page Load (when Using driver.get) or for Ajax PopUp Load Like let say I am entring something in Edit Box and some Look up or Ajax call is happening.

2. Where To Use Implicit wait

   Should I use after all the methods wherever Ajax call or Page load happening or only once it is enough (I am just taking the reference from Selenium RC where We can Use Selenium.SetSpeed Method).

Thanks, Arun