The IRS "recommends" you keep all documents pertaining to your returns (cancelled checks included) for at least 10 years. However, what they don't tell you is this: ~if for any reason~ they suspect you have filed a fraudulent return, they may examine your documents for as many years back as they choose. So, it behooves you to keep cancelled checks that pertain to your returns forever. But I would thin out what you no longer needed (or stuff that wasn't relevant to your return) and throw those away.
Hope this helps,