You can crack any password with enough guesses, but a good password will take a considerable time to guess. If you know that the password is a dictionary word (there are around 200,000 words in the full Oxford English Dictionary) and if it takes say one second to try 100 passwords.
Then it will take 2,000 seconds, or 33 minutes, to find the password.
If a password is a random string of letters, then the number of possible combinations is the number of options multiplied by itself for every character. For example, a five-character string of lower-case letters (26 options) could be any one of 26 * 26 * 26 * 26 * 26, or 11,881,376, possible passwords. At the same rate of guessing, this would take 118,814 seconds - 33 hours.
It will take 26 times longer for every digit added, so a random siz-character lowercase password will take 35 days to guess, and a seven-character one will take two-and-a-half years.
