At most, an astronaut without a suit would last about 15 seconds in space before losing consciousness from lack of oxygen (that’s how long it would take the body to use up the oxygen left in the blood.) Of course, on Earth, you could hold your breath for several minutes without passing out. But that’s not going to help in a vacuum. An astronaut who fell unconscious from lack of oxygen would last for a few minutes more before perishing from asphyxiation or the effects of the pressure reduction.

Source: SLATE