Anytime really. I can't think of a specific intent that would change that.
annualcreditreport.com is the home of the required by law free copy as mentioned. TransUnion, Experian, Equifax owe you a free one every 12 months.
I pull one every 4 months. Example:
8/1 - Experian
12/1 - Equifax
4/1 - TransUnion
Repeat the cycle. Be your own credit monitoring system.
Another tip: Check all accounts daily. Friend of mine works in Computer Crimes. His advice is that which I have taken to heart. As he also said, "how big of a head start do you want to give the thieves?"