Another way of saying it would be:
"What time are you open til?" (Spoken English, heard 44% of the time)
"What time do you close?" (Written and spoken, heard 53% of the time)
"Til what time are you open." (Most formal, heard 2% of the time)
The remaining 1% of the time consists of mumblings you can't understand, by drunks asking when they can get their last beer.