Assuming you married during that 90 day period, you can still apply for AOS. However, you will be "out of status" during any period between the expiration of the 90 days and the receipt by USCIS of your AOS application and technically subject to deportation from the US. Our advice to our K1 AOS clients is always to ensure their AOS application is received by USCIS before the 90 day stay expires. Once the application has been received by USCIS, AOS applicants are granted a period of authorised stay in the US until a decision has been made on their AOS application.