Depends on the reason you put the spare on. If it's bald and dead, then the rest are probably close behind. Replace 'em two at a time, if not all four.
If it ran into a nail or some other unfriendly artillery, then see if it can be repaired. A repaired tire can still last quite a long time.
I'll echo Felix on tread life. If you've still got decent tread, go with the tires you've got.
Oh, and once the flat is fixed (assuming it can be saved and is worth keeping), put it back on and put the spare back in the trunk. Keep it fresh for the next time.
And keep the number of a good tire place in the car, just in case...
