I'm currently under AOS. Married a USC 1.5 years ago, everything still pending.

When can I apply for U.S. citizenship?

You can apply for US citizenship 2 years and 9 months from the date you become a permanent resident. From your brief description, it sounds like you are still waiting for your green card interview..........I would concentrate on that first before you start thinking about becoming a citizen

Thanks for your reply, Paddy.

For some reason I thought I could count the wait necessary for citizenship from the date of marriage, not since I get a card issued. Yes, I'm stil waiting for the green card interview. If I happen to get a conditional card first, can I use that issue date to count my 2 yrs 9 mos, or do I have to wait until the 10 yrs card is issued?

Yes you can start counting 2years and 9 month from the date your conditional card was issued.