I think it's up to six months, because I received a check after 4 months of his delevery and I cash out the money without any problem!
If you're on about how long can you leave the cheque, lets say, in the kitchen drawer... then it runs out after 180 calendar days - after that it's void.
I think US bank issued checks are valid for a maximum of 180 days. However, the bank can issue checks that are valid for less than 180 days. If nothing is mentioned in the check, it is good for 180 days.
AdSense checks expire six months after the date that appears on the check. Source: https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=53067