From:
http://cxliv.org/2006/04/05/password_cracking_speed.php
Using this as our guideline, a 6 character password using the 62 characters possible from upper- and lower-case letters and numbers will produce 57 billion possibilities, and according to this site, Class D hardware can exhaust every such possibility in just 1.5 hours. That's not long. Add common symbols to that mix (increasing the possible characters to 96) and you increase the combinations to 782 billion (for the same 6-character length), meaning the same hardware will take slightly longer to go through all iterations: 22 hours.
Increasing the length of your password makes it more difficult - an 8 character password drawn from the pool of 62 possible characters (letters and numbers) means that there are 218 trillion possibilities, which would require the same hardware a whopping 253 days to process. Meanwhile, using the 96 possible characters that include common symbols would take 23 years on that same hardware!