Microsoft offers an excellent guide on making sure your passwords are secure, I’ve heard from other sources that you want your password to be at least 8 characters.  They kick up the requirement to 14 characters, but with 8 characters and following their other tips, like using a combination of upper case, lower case, numbers and symbols (!, @, $, etc) and avoiding easy to guess information like anniversary, birthday, etc 8 characters can be pretty secure.  Check it out here or check out some snippets below:

Keys to password strength: length and complexity

An ideal password is long and has letters, punctuation, symbols, and numbers.

• Whenever possible, use at least 14 characters or more.
• The greater the variety of characters in your password, the better.
• Use the entire keyboard, not just the letters and characters you use or see most often.

Test your password with a password checker

A password checker evaluates your password’s strength automatically. Try our secure password checker.

Protect your passwords from prying eyes

• The easiest way to “remember” passwords is to write them down.
  It is okay to write passwords down, but keep them secure. See 5 tips to keep your passwords secret.

Common password pitfalls to avoid

Cyber criminals use sophisticated tools that can rapidly decipher passwords.

Avoid creating passwords using:

• Dictionary words in any language.
  Words in all languages are vulnerable.
• Words spelled backwards, common misspellings, and abbreviations.
  Words in all languages are vulnerable.
• Sequences or repeated characters.
  Examples: 12345678, 222222, abcdefg, or adjacent letters on your keyboard (qwerty).
• Personal information.
  Your name, birthday, driver’s license, passport number, or similar information.