According to the code, a personal check becomes "stale dated" six months after you write it. If they weren't, "pay day" lenders and other crude forms of credit couldn't exist. Banks will often try to collect this from the writer, if possible.
If you're writing a check quickly, slow down when you get to the signature line. You don't have enough money in your available balance. Your check could also bounce if you filled it out incorrectly in other ways.