After 90 days, you are out of status....and deportable. Marriage to US citizen gives no immigration rights or rights to remain in the US. However, once you file a proper I-485, you are granted authorized stay in the US until the I-485 is adjudicated. "Authorized Stay" is not a legal status, but prevents further unlawful presence.
It is highly recommended to file for adjustment of status before the end of 90 days after entering the US. There is no benefit in waiting to do so.