I think the title says it all. Tell your current AP amount; X. Then tell your amount of days you been playing; Y. After that divide; X/Y=average each day!
The last time I checked (which was August 4th) it was 17.5 comments a day. Let's see what it is now. I currently have 6,029 AP, and It has been 340 days since I joined AG (Wow, only 25 days left until it has been a year since I joined!) therefore: 6,029/340= 17.732352941176470588235294117647. I now have gained approximately 17.73 AP a day.