(What time you hit the bakery?)
In old-school atavistic Mexico, probably one of the few chances a decent demoiselle would have to an encounter with her lover would be while performing her daily duty of getting fresh bread for dinner (particularly true if she worked as a maid).
Nowadays, the expression delivers metaphorically the same message: a prearranged, brief and furtive romantic encounter (the best kind).
A que hora sales por el pan mi reina: Let's meet my queen, set the terms.
Yelling it to random women means you dig them, and would also mean you're a tasteless stalker.