I do not know when it last was!! But I did buy a voucher other day on groupon for a professional oven clean!!!

My theory on the things in oven is that it gets heated up so hot that it kills all (I am not even thinking about bugs that like warmth to multiply))\

I always plannedon cleaning it but the oven cleaners i have looked at say dont use for 24 hrs prior or 24 hrs post using and seemed to hard to find a time when working full time, then didnt want to do when preg and then didnt want to do with kids in house so never got done!