Hello Sossa,
police certificates a valid for 6 months. So if you are in Kenya and your interview is in Nairobi, then you MUST have a police certificate that is not more than 6 months. Also remember your visa validity period will be based on among other things the police certificate period of validity. So if you go for your interview with 1 month left for the police certificate to expire, then you will need to travel with-in that period. You need to get the Police certificates about a month from your interview date. That way when you go for your interview and get the visa you can still wait in Kenya and travel after 4months and your Police Certificate will still be valid for traveling to the U.S.
If at one time you lived in a foreign country say Uganda and you get a Police Certificate from Uganda, then that one is valid for 1 year provided you don't ever return to Uganda with-in that year.
I hope this helps...