Password cracking programs typically use the dictionary attack or the brute force method to break your “strong” password. And you don’t really need a super-computer for guessing passwords, just enough time depending on how strong or weak the original password is.
If you like to know how long will it take for an average home computer to guess your password, try the Hackosis calculator. Just type in the count of various characters (lowercase, uppercase, numbers, etc.) and the tool will give you a time estimate.
For instance, it will take around two months for your computer to guess a reasonably strong password with 5 lowercase, 2 uppercase and 4 numbers (see screenshot above) but more common passwords like test, 123, password, etc. can be cracked in negligible time.
In case you like to use the password tool offline, get this spreadsheet.
Related: Retrieve Passwords in IE and Firefox