How "Real Time" is Google?

[Jordo needs a drink]

Once you see googlebot crawl a page you've made a change to, how long does it take for the changes to show up?

I know not all datacenters are updated at once, but for testing purposes:

1. How long does it take the first datacenter to update?
2. Generally, how long does it take to migrate to others?
3. As a rule of thumb, how long should you wait for seeing the result prior to making another change?
4. How does this compare to other search engines? (Should you wait longer to see the effects on others?)