They will have to give you the NBBO or better when off exchange markets are open. Spread is usually very small, only 5 to 6pm spread are up to 10 cent in spy (no Futures trading to hedge).
After hours is from 4pm to 8pm, premarket to 4:00am to 9:30 am.
If they offer a spread >10 cent (which is more than likely) from 8pm to 4am i would avoid it and trade ES or MES instead...