In most cases a personal check is valid for 180 days from the date it was written. If the check is not cashed or deposited by then, it is considered void. Most banks will refuse payment of the check if it is presented after this period.
There is also an option to notate how long a check will be valid for. Some checks will have a statement that reads that it is only valid if presented within 90 days. This would be shorter than the standard time frame, but most banks would honor the written statement and refuse to pay the check if presented.